In canasta. Can you meld three wild cards and two jacks to pick up a jack on the discard pile?
Yes, assuming that your total point value of the meld meets your minimum. From various sites that gives rules for Canasta:
A meld must contain at least two natural cards and not more than three wild cards.
Your meld would have 3 naturals along with 3 wilds, which meets both criteria.
It does not matter what the meld looks like not including the top of the discard. The top card of the discard is part of the meld, so you include that when seeing if the meld is legal. This is true both for determining if the meld meets the minimum points and if the meld has enough naturals.