I would say if you're not using a metal mag catch you definitely need one or two of them. Also an ejector in case you lose yours. Buttonheads, extractor, extractor spring and frame pins. I personally like to have extras of all the springs. I have yet to replace any yet but Kel-Tec is so generous with parts I figure why not. I have paid for some parts but most were free.
I would say parts #181, #182, #183 (extractor, extractor spring and bolt), #115 (ejector), #205 and #208 (mag catch and spring), #191 and #191 (recoil springs), #271 and #275 (hammer and hammer spring).
New extractor parts may fix failure to extract issues (FTE).
Buttonhead screw, AKA "frankenbolt" is easily stripped.
Ejector is easily lost.
New mag catch and spring may fix mag retention issues.
New recoil springs may fix functioning problems, besides, the do wear out.
If your hammer spring breaks you're out of business until you can get a new one, rare, but it does happen. Its happened to me. Kel-Tec sells the hammer and hammers spring as a singe unit.
Hi, The part that I see coming up all the time on this site is the mag catch. Since it is plastic, you should press the mag catch release in when inserting the mag and also when you are removing the mag by pushing up slightly on the mag and then pushing on the mag release to drop the mag. Never dry fire. If you bought it new you have a life time warranty that can't be beat. I have 2 P3AT's and never had one single problem. They are both a generation 2. I am sure others will give you additional assistance on the subject. Good luck !!