There are two questions on Holon Farmer.
One has an answer, the other is being double checked.
First, you can't take back the card that you discarded.
Not an answer that I'm happy with, but that's the official word on it.
Second, it's being double checked if you have to take 3&3, or if you can take less if that's all you have.
You certainly can't take less than 3&3 if you have 3&3 in the discard, but we're rechecking if you can take "all that there is" if you have less.