Alex goes to draw for his turn and accidentally grabs two cards.

He looks at both, but they don't get to the other cards in his hand.

He says “oops” and then proceeds to put the bottom of the two cards into his hand and the top one back onto the library.

In effect, he drew the card that was second from the top after looking at both.

Is this UC - Cheating?

Would it matter if it was turn 3 and he pulled the 3 drop putting the 4 drop back on top?

This sounds very much like text book Looking at Extra Cards.

Remember, in order for someone to have committed Cheating:
(1) The player must be attempting to gain advantage from his or her action.
(2) The player must be aware that he or she is doing something illegal.

Here it sounds like the player made a mistake, and made an honest effort to fix it. It's the kind of thing that happens a lot at Regular or with newer players.

As for the part about 3 or 4 drops, the same applies. Of course, the specifics might make you suspicious, but all that means is you do a little investigating to see what happened and go from there.

Like most Cheating scenarios, it depends: investigate, and if you believe that Alex knew that was wrong, and did it to gain advantage, then UC-Cheating does apply.

In your example, it might be quite difficult for Alex to convince us he didn't know there was anything wrong with what he just did.

However, it is conceivable that he thought the appropriate “fix” was just to grab the one he would have drawn - and was confused about the ordering. In this case, it may be that Alex wasn't trying to gain an advantage.

One key point to keep in mind: if Alex is just trying to fix it himself, and avoid any penalty from a Judge, that is seeking advantage. (The advantage is not getting a Game Loss, or even not getting a Warning.)

d:^D