The more inserts you use, the more vibration you get.

Given the way the drivetrain is mounted, most of its motion is in a fore/aft direction. The first mount I would add the insert to then, is the front one. Furthest away from the cabin and will help to limit the motion of the engine somewhat.

If you do one side mount you should do both, otherwise the one without the insert will be subjected to a whole buncha torque because the motor rotates around them and limiting the force on one side will transfer it to the other. Adding the side inserts will give you a lot more vibration, as you have noticed.

I would see how you like it with just the front insert.