Hey there,
CRs aren't additive. For example, two CR 3 creatures isn't a CR 6 encounter. Normally when you have differing CRs you kind of just "go up a level". For example, the goblins and warchanter would most likely be a CR 1.5 taken together. The other thing to keep in mind is that if you're running a group of 4 PCs, you're also on the low end of player power, as most encounters are for a group of 4-6, so something that's pretty easy for a group of 5 gets a bit harder for a group of 4.
But CR is measured assuming a party of 4 or 5. The encounter creation guidelines state it pretty explicitly.
The trouble is that most of the time, A CR=APL equivalent encounter is pretty close to a nuisance encounter: no real threat, no real excitement.
PF characters tend to be pretty tough. They can handle the fights. If they can't, you mod on the fly.
Yes. CR determination Is art, not science, but three CR 1/3 encounters would be about a CR 1, and and a CR 1 plus a CR 1/2 would be maybe a CR 2.
And look at the damage they deliver. Not nearly as much in one shot as, say, an Orc with a greataxe, who has a very good chance of taking out even a level 1 fighter in one blow and will almost certainly kill a PC if he crits.
Also note that these are goblins, not tactical murder machines. If you feel the PCs are losing, just have thm run crazy or something and go try to kill a random civilian. Also they are unlikely to try and finish off downed PCs. There are too many crazy things to do.
The idea is to get the PCs introduced to the craziness that is goblins, not kill them. If things go really bad for the PCs, have Sheriff Hemlock or AldernFoxglove or some random civilian with a big stick show up to help them.
The other thing to remember is that a CR 1 encounter is not an even match for four level 1 players -- it's supposed to be a fairly easy win for them. So if you want it to be difficult and risky, you have to bump the CR up a bit.
If you want to make an encounter tougher, it's usually better to add more monsters (as long as they're effective) rather than to make the individual monsters nastier.
It gets easier as you go along.
Reiterating that playing the goblins as crazy as they're supposed to be helps make these encounters both easy and memorable.
I let my kids play the goblins. They spent more time lighting each other on fire, trying to destroy the scenery, and running away from the earthbreaker-wielding barbarian who was one-shotting them to be any real threat to the PCs. It was hilarious.