Feelings of inequity are understandable. After all, the four if you are human, and humans are imprecise. Even if D is precise in what he gives the three of you (your story says otherwise), your individual perceptions may highten differences.
As for the question if mono or poly, you're in a poly situation. If it quandary like a duck and has multipleoving partners like a duck, it's probably a poly duck. You may be mono-inclined poly or poly-friendly mono, but the important part is you're looking for answers.
The question I have for you is simple: what do you want to achieve? Where in life do you want to be? What does your future look like with D? And how dies it look if he were gone tomorrow?
I take that back. Thise aren't simple.
Your case sound like a classic poly tug-of-war. There's obvious hierarchy, but it's confused by mixed messaging. Know where you want to be could help thus group get you there.