Multilateration with 3 receivers

Hi, Ive been battling with multilateration all day today, and I just
cannot get the maths to work. I know that normally multilateration
uses 4 receivers, but when you know one of the lenghts, usually depth
or hieght, you can use 3. The only real resource I have found is in
wikipedia, but they just say "solve for the unknown target location".
Ive been trying to solve it, but cannot.
Does anyone know of any better resources I can get my teeth into?

Thanks,
Phil Winder
.