If you cast Life Siphon on 3 different creatures, and you have it so it's -2(them) +2(you), each creature has -2 degen and you get +6 regen. They stack.
However if 2 necromancers both have Life Siphons that give -2/+2, and they both cast it on one creature, I -think- the creature only suffers -2, but each necro gains +2. I'm not sure on that part though.
But yes, if you spread it around by yourself, the regens stack.
Your rank in blood magic has to be at least 12 before you get the 3 degen/regen.
And yes, you can cast it on multiple foes. The regen to you will be cumulative.
Why bother at that point, just do Life Transfer for -6/7 degen + 6/7 regen
Because Life Siphon can effectively give you the same regen that Life Transfer gives you, 3 degen on multiple targets and still allow you to bring a more effective elite.