2 lbs per month is not normal
3 lbs per month is significantly abnormal
Unless you lived in an extremely cold environment, which you do not.
We left our Model S to sit for six months this year, in a controlled atmosphere (65-85ºF). Upon returning, all four of its tires had dropped from 45 to 36psi (two) and 35(two). That was acceptable to me.
You, however, likely are more consistently driving your vehicle and I certainly hope are checking tire pressure more frequently than that - or perhaps your vehicle is a newer model that displays the pressure. You CAN realize greater air loss if the roads you frequent subject you to greater than average amounts of road irregularities: a pothole, for example, can bounce the tire + rim sufficiently to squeak out a little bit of air.
Hope this helps.