Rod,
On your D3, I think it is on you accessory tank. At least, I think that is where ours is located. To me it doesn't make sense, but that is where they put it, I think. Basically, the two needles on your gauge show brake pressure. The low air alarm should be on you accessory tank, if yours is like ours. That will be under your front Jbox, lower panel, next to your spare.
That being said, Clifford knows what he is talking about. So I just offer another place to look, not a different place.
John