Put the green block onto a family. For each green block, check if overlapping family at all four orthogonal positions. If it is, add 1 to a counter variable. If the counter variable is greater than 2, delete the green block.
Of course, this changes if the next block you check might have had more 3 adjacent blocks before deletion, so you've got a game logic issue to solve even before implementation. If you place a block that results in two adjacent blocks needing to be deleted, which one gets deleted?
You might be able to get around this by flagging the block with another instance variable instead of deleting it right away, then delete all flagged blocks at the end of the tick.