The Delta card supports zero latency monitoring (as does my C-Port), so here's how I do it (it should work the same way with yours):
1. Record your soundtracks (say, tracks 5 and 6)
2. Plug in your 3 microphones into channels 1, 2, and 3 of your mixer
3. Cables out from mixer via channel inserts 1, 2, and 3 into Delta's breakout box (1, 2, and 3).
4. Cables OUT from Delta (1, 2, and 3) into mixer channels 4, 5, and 6. That's the zero latency monitoring part. You'll probably have to "assign" the outs somehow via the included software (that's how mine does it, anyway). If you have the tip/ring soldered 1/4" cables, you can skip this step altogether. I haven't done it yet, so I use the sound card for monitoring.
5. Cable out from your Delta's main output into mixer channels 7 and 8. (I like to use a cheap A/B splitter for this - it's much more convenient, but not necessary)
Set your levels and you're ready to go. You'll have 3 separate vocal tracks to mix independently.
If your mixer is like mine, then you'll use each channel's trim knob to adjust your input levels to the Delta card, and the fader to adjust the levels in the headphones (my channel inserts are pre-fader).
Hope this helps, and I haven't made it more confusing than it needs to be

Just remember you can skip step 4 if you have (or make) the right cables.
As far as recording the soundtrack onto 2 channels - that's the easy part. You just need a "Y" adapter cable (1 stereo jack to 2 mono jacks). Most electronics stores and music store have them.