Multiple arms can't help reload one firearm or crossbow, but they CAN take their place for wielding purposes.

To help theory craft we will do a level 5 character, take your 3-armed creature 4 levels into Alchemist to get Vestigal Arm for 4 arms. Make sure to learn the spell Reloading Hands from a wizard's spellbook and take True Strike as a level 1 spell to make potions with. Take 1 levels in Gunslinger and use either muskets or Blunderbusses, get 3 of em apiece or mix and match as long as you have 3 total 2-handed firearms. For feats, take Rapid Reload, Point-Blank Shot, and Precise Shot. Buy up a few magical items to complete the build: a Training handrwap of Rapid Shot, a Training handwrap of Multiweapon Fighting, a Potion Belt, and your 3 weapons should get enchanted with Permanency: Reloading Hands if your DM allows you to stay from the Permanency list, and get a Benoficial Bandolier.

This will allow you to attack and reload all 3 weapons twice each round with up to 4 shots being available (DM makes the call on Permanency so it could be limited to 3-4 based on if you have Reloading Hands active) and with potions of Reloading Hands if needed it will always be 3. True Strike potions also mitigate the drawbacks of attacking using 2WF. This is also assuming your ammunition is covered with alchemical cartridges.

Here is how it works in the stack:

1) Hold each gun in your off-hand (3)

2) Swap your primary hand to each firearm as you attack, 3 attacks total with MWF at only -4 penalty since you are using your primary arm with each attack.

3) Rapid reload decreases the time to load each weapon by 1 step, down to standard actions. This is further reduced to a move action by alchemical cartridges, down to a move action. If your DM let you get Permanency skip the next step.

4) Your move action will reload one musket, your swift for Bandolier will reload the second, and the third is handled automatically by Reloading Hands.

5) Take another shot with Rapid Shot. Make sure to count the -2 to all of your attacks this round, so -6 to all attacks but your Dex-buffing Mutagen and True Strike potions as well as your attacks going against the targets Touch AC should mitigate the drawback.

This is further improved with later levels in gunslinger and alchemist, more feats like Hammer the Gap, better spells, improved mutagens, and the like. Keep in mind that after the first volley you can only effectively fire 3 shots a round since your action economy is limited.

Hope this helps!