Monster Creation Advice

[notty235]

Now, I'm running a 3 players campaign. I'm going to create a dungeon for them but I do not sure about the monster in the dungeon.

I want to know what CR of the monster they will able to fight not to difficult.

And what CR of the monster they will have a hard time with it.

The party have a sorcerer, a cavalier, a brawler and a swashbuckler NPC that now injured.

[Drone]

Add their levels together then divide by the number of player characters (if they're all the same level, just use that).

An average fight would be a monster that had a CR equal to that number. There's only three player characters (I never count NPCs since they're usually under-equipped compared to the rest of the party) and CRs are designed for four players so you have to come down a bit.
Without a healer character, fights are even tougher, but I think this is a good measuring tool.

For example, let's say the Average Party Level (APL) is 3. About half the encounters should have a CR 1 lower than that (CR 2) while a boss monster encounter should be 1 or 2 higher (maybe CR 4 or 5). Anything higher than that and you're looking at possible character death(s). Also keep in mind the characters' weaknesses. A CR 3 shadow is tough against 1st-level characters but it's a party-killer if no one has magic weapons, force spells, or the ability to channel positive energy.

[Jericho Graves]

it depends on the level of your characters

[master_marshmallow]

Add the levels together and divide by 4, that is APL.
If they are using point buy or have naturally low stats, that is the correct CR to use. If they have better stats then you can add one or two to APL and have a challenging fight.