If I use Double Trouble and do not reveal a unit, do I put the 3 cards back to the top of my main deck?

No, you do not put the cards back on top of your main deck.

When you resolve Double Trouble, the instruction "Recycle the rest" means that any cards you looked at but did not draw must be moved to the bottom of your Main Deck.

According to Rule 416.1, "Recycling cards is the action in which a player takes one or more cards from a specific zone and then puts it on the bottom of the corresponding deck." Because the cards are being recycled to your Main Deck, they go to the bottom, not the top.

Furthermore, Rule 416.5 specifies that if you are recycling multiple cards simultaneously, they are placed on the bottom of the Main Deck in a random order.

Note: I could not fully verify this ruling against the retrieved rules.