You can have that effect with Reactions. "During your action phase, you may reveal this from your hand. If you do, play it." You may add a keyword for that if you are planning on including into several cards (however, the fact that "Each other player gains a Curse." has no keyword seems to point in the opposite direction), but I see no reason why would you need another type. The text is not so long.
Strictly worse than Bazaar without the attack but somehow too similar so it shouldn't cost 4$
5$ Action - Attack - Reaction
+1 Card
+1 Action
+1$
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During your action phase, if you have at least 3 actions in play, you may reveal this from your hand. If you do, play it. If you have at least 1 attack card in play, each other player discards down to 3 cards.
