Modifications? What do you mean?
The game has rules for the sort of things you can do, like magical upgrades or adding bayonets.
But your post here doesn't seem to be talking about those items.
Do you mean the Weapon Modifications stuff? That's a pretty minor thing, mostly it's just Versatile Design, very few characters use that stuff.
Pathfinder is a fantasy game, so it shouldn't be much of a surprise that most weapon improvements are magical in nature. This is the list you're looking for, but unless houseruled, you need the Craft Magic Arms and Armor feat for that, for which you need either caster levels or the Master Craftsman feat.
Although in a pirate themed game, I would expect to find plenty of firearms and ammunition on enemies...
I would ask for a rifle but I don't want to use one considering 2 out of 5 of my teammates don't think guns should be in pathfinder (despite us being on a pirate filled with flippin cannons which are literally just massive guns) and I feel that it would be a bit to op due to how the increments and touch AC work. Magical enhancements could work considering most of my team are freaking magic casters so one of them **could** possibly enchant but knowing my DM i'm most likely going to be on my own.
My hope is that I manufacture a triple barrelled musket (reload is a free action for me) with either magical or (most likely) physical enhancements. The design i'm thinking of is almost steampunk like. Imagine a the barrel having a triangle like design with bayonets poking off the edges of them. A mechanisms for attaching and detaching modifications as needed (silencer, flash muzzle, etc.). I would also want railing for a scope. Something that may border the lines of modern is a switch that allows the gun to be changed from single fire and a single three shot.
I understand that all of this would require high af rolls and a DM to allow it but most of this is just what I hoped to END up with at like level 10 or 12 (most of our campaigns end around that level)
I think most of what you're wanting isn't actually upgrades to your gun, you think they are because you're used to modern technology, but there are methods to accomplish what you want, but it's all magical.
For example:
This sight can be attached to a single two-handed firearm. When this is done, the sight becomes part of the weapon, but can be removed from that weapon with a full-round action. A firearm wielder can choose to spend a full-round action to make a single shot with a firearm that has this sight. When she does, she can resolve the attack against the touch AC of her target regardless of the range increment.
When applied to a one-handed or two-handed firearm, this strange grayish oil renders that firearm silent for 1 hour. Five vials of oil of silence can be used in conjunction to silence a Large firearm siege engine, and 10 can be used to silence a Huge firearm siege engine. The oil does not work on firearm siege engines that are larger than Huge.CONSTRUCTION REQUIREMENTS
So there are rules for it, but they're not gun modifications. And you can't make them as a non-caster.
And, even if your GM we're to allow you to make them, you would still be advised to use the items already written.
Also, there are other items written but I'm not going to look through them all for you.
Even if your GM allowed you to create these items without having the feats or abilities to create magic items, even if they allowed you to create it only using your craft gun skill, the price should still be maintained the same as the magic item version.
Why? Because the game is balanced around these items costing a certain amount, because wealth is as much a power track as experience/level is.
As far as I know, there is no such thing in the Pathfinder rules as a tri-barrel musket. There is a double barrel musket.
This musket has two parallel barrels; each barrel can be shot independently as separate attacks, or both can be fired at once as a standard action (the attack action).
If both barrels are fired at once, they must both target the same creature or object, and the gun becomes wildly inaccurate, taking a –4 penalty on each shot. Each barrel of a double-barreled musket uses either a bullet and a single dose of black powder or an alchemical cartridge as ammunition.
There is nothing about this musket that allows it to be reloaded faster than a normal two-handed firearm. Advanced firearms, which would be things like rifles do reload faster. You can also take rapid reload, special ammunition, and the musket master archetype of gunslinger to reload two-handed firearms more quickly.
However, to my knowledge being Kasatha doesn't reduce the load time and wont get it down to a free action.
If you are a musket master with rapid reload who uses alchemical paper cartridges you will be able to reload as a free action I think, but you could do this regardless of race.
My hope is that I manufacture a triple barrelled musket (reload is a free action for me) with either magical or (most likely) physical enhancements. The design i'm thinking of is almost steampunk like. Imagine a the barrel having a triangle like design with bayonets poking off the edges of them. A mechanisms for attaching and detaching modifications as needed (silencer, flash muzzle, etc.). I would also want railing for a scope. Something that may border the lines of modern is a switch that allows the gun to be changed from single fire and a single three shot.
I'm sorry, but this doesn't even remotely sound appropriate for a pirate setting to me. The silencer was invented in the 20th century, scopes weren't intented before 1835, flash suppressors weren't even needed before the arise of shorter barrels in the 20th century. I'm honestly a little surprised you asked for a bayonet (albeit multiple ones, which makes about as much sense as duck taping multiple knifes together to better stab someone with), and not a chainsaw add-on.
Plug bayonets do at least exist in Pathfinder, but I'm pretty sure you're looking for socket bayonets (which were at least invented near the end of the 17th century).I'm not trying to be mean here, but you should really ask yourself if your ideas are setting appropriate. Especially when other players already dislike firearms (and your GM not only allows you to play an overpowered race from a different planet, but also made fairly major houserule in your favor, removing one of firearm's biggest downsides), maybe you shouldn't try to push towards more modern firearms.
Firearms in Pathfinder (at least early ones) are blackpoweder weapons which make them nightmares. Slow to reload, low capacity, unreasonably expensive, and not much better than a crossbow in the hands of anyone except a Gunslinger.
I completely rewrote the gunrules for my personal campaigns, but that doesn't quite help you so lemme dig up the original rules.
Ahem.
So, according to your wants you need a Cylinder Rifle which can fire up to 8 shots before requiring a reload. It costs 6k to buy, but you can build one for a mere 3k gold thanks to the Gunsmithing feat. While you cannot build many additions for your weapon, you can construct ammunition. About halfway down this page is a list of all the ammunition you can craft, including alchemical cartridges (note: Cylinder Rifles can't use these but they would be good on your pistol, consider building yourself a Pepperbox or Paddlefoot). There might be additional firearm accessories from various books, however the ones I found at a conventional glance all sucked eggs because of course they do.