Raid abilities evaluate the entire turn to see if you attacked with a creature. That creature doesn't have to still be on the battlefield. Similarly, the player, planeswalker, or battle it attacked doesn't have to still be in the game or on the battlefield.

Raid abilities care only that you attacked with a creature. It doesn't matter how many creatures you attacked with or which player, planeswalker, or battle those creatures attacked.

If there are fewer than three cards remaining in your library when Gutless Plunderer's last ability resolves, look at all of the cards in your library and follow the instructions in order. For example, if you had only two cards in your library, you would look at both, put up to one on top of your library, and put the rest into your graveyard. Sounds like a gutsy move.