A: The way this works looks like this:

1: You have your hand, and decide to play Draw 3 Play 2 of them (D3P2)
(You put D3P2 on the table, since technically it’s not yet in the discard pile)

2: You set your main hand aside, and draw three cards, creating a temporary sub-hand, or “mini-hand.” One of them is Draw 2 & Use ‘Em (D2UE) and you decide to play that for your first of the two plays.
(You put D2UE on the table, with the other, since technically it’s not in the discard pile yet either)

3: You set aside your mini-hand from D3P2, which is just two cards. They do not get combined with your main hand.

4: You draw 2 cards and use them in whatever order you wish, and then D2UE goes in the discard since it’s finished.

5: You then pick up your 2-card mini-hand, and finish D3P2, by playing your second card and discarding the last one. D3P2 is now completed and goes in the discard pile.

6: Then you pick up your main hand again, and continue your turn (assuming you have any plays left, of course).

There are a few other cards which can cause situations like this, where you are playing a mini-hand (or sub-hand) with a main hand set aside: Fizzbin and Goal Bonanza. Naturally, this means that they could also be involved in a “nested sub-hand” situation as described here, and hopefully you can extrapolate from this example how that would work.