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Say I have 3 Decepticons on the battlefield when I play Overwhelming Advantage. Can I put just 2 cards from my hand on top of my deck?

Yes. Each Plan is optional, so you could anywhere from 0 to 3 cards from your hand on top of your deck.
Source: Wizards of the Coast (2019-11-08)    【Link to Ruling】
Tags: Overwhelming Advantage