Advice on removing auto box
 

Im trying to remove an automatic gearbox from a 1995 WRX.

Ive got all the 14mm bolts off around the side and remved the starter. Its all off apart from the 3 12mm bolts holding torque convertor to gearbox. I got under to try and remove but the torque convertor keep spinning each time I try to turn bolts. I tried holding torque convertor with mole grips but still cant do it. What is easiest way to remove these bolts ?