I want to mount a 1.6 to an engine stand but the torque converter is still attached to the flex plate by three bolts / nuts.
I tried a 3/8 drive 14mm socket and broke the socket.
What's the trick to removing the nuts / bolts to get the converter off?

There should be no trick! Mine came out wuth no problem. I would say try again! Heat the nuts if you have to!