Triominos “take from the well” rules
You have play the triomino that you bought, you can't use the triomino that you already had.

From the rules:


  YOU HAVE NOT PUT DOWN A TRI-OMINO?
  
  If you cannot or do not want to put down a matching Tri-omino, you MUST 'buy' a Tri-omino. You may then play this 'bought' tile in the same turn. (emphasis mine)