If you're short on cash I'd say preorder D3 and wait for the release, it shouldn't be long now, and play D2 demo in the meantime.
Myself I did buy D1 two months before D2 went out but I had money and wanted to "help" Illwinter. Since then I've played so much D2 that I think I should have paid them *more* !
The real question is how long before Dom III is released. If we are looking at a couple months, the answer is simple. If we are looking at 6, 8, 10 months...then the answer is also simple, just different.