I noticed today that players 2 & 3 can't earn experience or achievements in Otomedius Excellent local co-op ("Multiplay"). Can they earn them in online co-op?
All three players can gain experience, weapon cards, and achievements while playing online multiplayer.
However, only the weapon cards shared (if they finish the stage). Experience and achievements (I believe) are gained based on each individual player's performance.
As a side note, if players 2 or 3 get lose all their lives, they are booted to the lobby until another player reaches a checkpoint. Like in local multiplayer, once the host (player 1) loses all their lives... it's game over for everyone! D: