For the ammeter to register power being used, the 3 must be fitted to the out side of the ammeter, or it will show negative drain.
Quite which is the out side you will have to find by trial and error. I would suggest as you look at the gauge, the right terminal logically should be the out side. Check to make sure the lead from the alternator is connected to the in side. This should be the brown with a white trace I believe.