3mm is quite good for blue mussel pearls! I've been collecting for many years, and only have perhaps 20-30 over 3mm with my largest being about 5x4mm.
As for what to do with them, that's really up to you, but I personally keep mine in a small, clear plastic box with a magnifying glass in the lid. You can buy them at the container store, and they're great for displaying pearls.
