I guess you already tried to configure SP3 for Google Voice via clicking on the gear icon and following the setup for GV?
On another point, something doesn't make sense in both of your posts. I don't understand what you mean by, or how you accomplish getting three GV accounts to 'work' on two SPs?
The way it works is one SP = one Google Voice number. Are you saying you have three different GV accounts configured to work on two SPs, SP1 and SP2? This is not possible, so something might be wrong here.
Or are you saying you 'switched them out' to test, for example you setup GV#1 on SP1, and GV#2 on SP2, and then you setup GV#3 on SP1, with GV#2 on SP2, etc. You'll never be able to have three GV accounts working under two SPs,