OR although Heaven and Hell indeed exist and are at war and, afawk, unaware of the existence of other parties, such as Trag'Oul, there are many more civilizations (as in Diablo both Heaven and Hell can be perceived as Extra-Terrestrials entities as well) other than them.
In fact there is no lore that prevents D3's "Pantheon" of being more than dual or bipolar. Thus, the Monk's 1001 gods can be just another entity exterior to Sanctuary.
Q u o t e: On the wind a shout
The barbarian strikes down
Flesh renders from bone