Question:
Not sure if it will make any difference in noise but:
If I'm running 3 10/2 round Romex lines, approx 40' each, is it better to keep them parallel to each other from panel to rack or to lightly twist the 3 lines together and run them as a "single" line?
Keep them separate. No reason to couple noise among them, even if the chance is small, and in general it is better to keep (potentially) high-current lines separate to reduce heat. And twisting will extend the length.
Curious: Where are you that you can run 10/2? Most places around here (USA) you'd use 10/3 or 10/2+G to provide a safety ground to meet code.
Code required 14 AWG for 15 A and 12 AWG for 20 A lines when we finished our basement a few years ago. I asked for 10 AWG to the 20 A outlets in the media room as the extra cost was in the mud compared to the overall cost. They did run a 100 A subpanel for the basement to cover the media room plus an extra 20 A outlet for our treadmill.