FWIW - The fact that your boyfriend is not your best friend, ultimately dooms the relationship with both of them, regardless of what you do from here on out.
You did not define what you mean by "long-term".
IMHO (and experience), long-term is measured in decades, not months or years.
Also - FWIW, male mammals are competitive by nature, especially in relation to relationships.
I recommend you rethink this - outside of the borders of the ego-trip that having two studs into you (so to speak) involves.
Chances are, this will not end well when it ends - and it will.