its really quite simple skys T&C's state that 2,3,4 or more boxes all need to be connected to the same phone line, purely so they can tell there all in the same house
1 box no phone line needed
so yes if you want a 3rd box it should be connected, but as you haven't done it with 2 boxes (i did the same for years) you might get away with a 3rd, try it not as though they won't warn you first in a letter
also depends on if you order a lot of films from them as without the phone line connected on a multiroom system you wont be able to use sky store, if you do.