I would bet dollers to doughnuts your coming out of posture.
Its hard to tell with out seeing your swing.
The three wood off the deck was the hardest shot for me to get consistant with. I had many bad memories to overcome before getting confident with it. It stayed in the trunk of my car so I wouldnt be tempted to use it on the course.
I read a tip on the Swing like Moe website that helped me the most.
I have a wide stance and when I came around it stressed my left knee causing me to raise up on my toes before impact. The tip was to flare the front foot out a little to relieve stress on the knee. Focus on keeping your feet flat on the ground until after impact. Also make sure you are standing far enough from the ball. Good luck !