Hi I'm new to doing diy jobs, but I already wired up a slave phone line to the master box. I'm now looking to wire a 3rd box up, would I wire this off the slave box or the master ?
For simple telephone service it doesn't matter. You can run them all to one point or chain from one to the next; whichever is easiest to run.
For more advanced use of the phone wiring like DSL, having the shortest total path of wire to the demarc point is preferable, with as few splices as possible, which usually means running a cable from each jack to near the demarc point, usually in an electrical room or basement.
Running them all to that point also means if a wire breaks it only takes out that jack, not all the ones downstream of it.