Was the bolt stuck completely closed, or slightly open (unlocked)? If the failure occured only 3 times out of 100, I'd suspect ammo first; or something slightly out of spec in the chamber (rust, foreign material, corrosion), that's not bad enough to affect every round, but bad enough to affect rounds that are at the large or long end of spec. Clean the chamber well before your next range trip.
What did you do with the 3 rounds that got stuck? Did you try to rechamber them? If so, did they go in the 2nd time? If not, did you keep them? If so, are there any obvious defects (out of round, etc)? If not, take them with you on your next range trip and try them in the chamber again. If they go in freely, fire and function, then your problem was a dirty chamber. If they won't go in the gun, then the problem is with the ammo.
A slight mark from a light firing pin hit sometimes occurs to rounds that are just chambered, with no attempt made to fire. It's the nature of the beast (free-floating firing pin).
YMMV. Good Luck and Good Shooting,