Radio propagation doesn't work anything like light from a bulb at this scale - its highly directional affected by all conducting metal objects that cause wave reinforcement, cancellation, reflections (multi-path reception). Particular sizes of conductor will resonate like aerials leading to strong scattering, and sources of external interference will cause issues too (accurate measurement of signal strength is not trivial).
You can rely on radio propagation to be line-of-sight in free space, but you still need to know the transmitter's polarization in order to be able to calculate distance based on signal strength, so a transmitter that's free to move/rotate in 3D still can't be measured this way - you only get order-of-magnitude information from signal strength.
What you can do is measure time-of-flight differences like GPS system does, should you happen to have the precise clocking setup between multiple receivers. This is more practical with ultrasound where the time delays are more practical, but multi-path reception is an issue with time-of-flight too - sophisticated signal processing is often needed to get good results I believe.
Setups with IR cameras fair much better, although its more complex the triangulation of an IR LED indoors works well - is used to monitor quadcopter swarms for instance, and has been taken to new levels with the Kinect.