Not exactly, though yes, the three versions of degeneration stack together
IE: A bleed, a Poison, and a Health Degen (IE: Necro or Mesmer spell) stack together, leading up to a decent amount of DOT
HOWEVER.
Multiple instances of the same type of DOT do not seem to stack. So if you cast a second phantasm or a necro casts a degen spell on the same creature, you dont get even more.
To be honest, in this case I am not sure which takes precidence: The higher degen amount or the last cast.
Anyone have a definitive anwser on that?