I'm working on a project where I have a GPS and two receivers to create a triangle with a transmitter. The problem I'm trying to figure out right now is how can I get the position of the two receivers?

I know it can be done, but I'm blanking on the math and Google hasn't been too helpful, maybe I'm not searching it correctly.

Here's my circumstance:
GPS is reading lat: 42.331429 and lng: -83.045753. My device is sitting perfectly level and pointing due north. My two receivers are 30 inches to the left and 30 inches to the right. Can anyone help me figure out the equation to get their positions? I know these are going to be very small differences, but that's as far as I've gotten so far.

Also, if things were more complex, how would that be solved? Such as pointing North/West, and 5 degrees rotated on each axis, x, y, and z? I realize that's kind of a crazy scenario, but I like to potentially prepare for every scenario. These are based on being in fixed positions from the GPS and the plane that the GPS is on.