You didn't detail how to determine where the transmitter was in relation to a single receiver. Since we're talking about 40cm, time delay probably isn't a viable option. Did you leave out pertinent data, or is that the question?

Oh, wait. I think I misunderstood your question. You're saying that you're currently able to get distance from each receiver, right? And you just want to calculate the absolute position (well, relative to some arbitrary point) from that data, right?