It refers to a card that contains the specified text or icon in its name, traits, effects, inherited effects, (Rule), digivolution requirements, DNA digivolution, DigiXros requirements, burst digivolve, App Fusion, Link, or Assembly requirements. For example, a card with [Knightmon] in its text would include cards with the name [DarkKnightmon] and cards with the text [Knightmon] in their effects.

The effects trigger simultaneously, so the player can choose the activation order.

Yes, you can. This card is treated as an Option card with the [Three Musketeers] trait.