I have had Rel’s for 20 yrs so This is my guess.. One sub is connected red to +L & yellow to +R so you get both channels into the one sub. Two are hooked up in a stereo config. with one serving left channel and the other the right. I believe there are possibly 2 ways to connect 3 Rels.
option 1 - Connect all 3 in a config like the one in which only one sub is used.
Option 2 - If you have the 2 connected as a stereo pair, then the third would be connected to both channels as if it were a single.
This way you still get both channels into the 3rd sub. I cannot see any harm to come from this and have also been considering this since I have a Storm 3 which I am not using now.
EDIT Depending on the placement of the 3rd sub, option one may be the best balanced approach. This way all three will be receiving a stereo signal.