Three wheeler pulls to left when riding

From your limited description it sounds like the left rear wheel is slowing and pulling the trike to the side. I would think this is caused by unintended friction, especially since you state that it worsens when going over a bump. I'd check for any rubbing brakes or guards on that side if you haven't already. It might even be a bearing problem, though I imagine if it were bad enough to pull noticeably you'd probably be hearing noises from it too.

Trikes with fork rake lean to the right on the camber of the road. They naturally try to steer left as the trail of the front wheel's castor forces it so. At least in drive on right side of the road countries. PIA. See if it wants to steer the other way if you ride it on the wrong side of the road. If doesn't cancel out you've got something bent.