If you know how to calculate sin, cos and tan then you can reduce the 3D problem to a set of 2D problems and solve them simultaneously. It's not rocket science. Well, technically it is. But it's not especially hard - it just needs you to solve some simultaneous equations. I'm too lazy to solve them for you, though. If you can use Processing's 3D rendering engine to get it to do the calculations for you, that strikes me as a very sensible approach to take. I suspect that in order to get any sort of accuracy you'll need to use successive approximations anyway, so the accuracy of the initial azimuth/elevation calculation is not critical; the angles you're giving as input are themselves only approximate solutions to the real problem of hitting a specified landing spot.