Dedicated circuit

To me 3 wire is,for example, 12/3 w/ground.
so I could run 12/3w/ground tie the hots together and end up with an effective gauge of 9

Unfortunately, the ac compressor is going to infect the whole house, it won’t be confined to the subpanel. Running a 100 foot branch circuit makes it a magnet for EMI generated throughout the house, so I would use 3-wire romex. The wire is pre-twisted and helps with rejecting common mode noise, which could be a problem at that distance.

So, I assume using 3 wire I would combine the two hots, giving me an effective gauge increase.
At this point I am only installing one outlet for my amp.

Wish I could have chimed in earlier and clarified things before assumptions started.I do thank all who replied.  First off it's an air compressor. My stereo setup is in my shop. I build hotrods so I apologize for just saying compressor. It's a garage thing I guess.I have a 200 amp main. Subpanel feeds most of my shop: T8 lighting, outlets, 220 for air compressor, some kitchen lights and an outlet or two. kitchen appliances have their own off the main.I don't need ultra highend hookups, just a solid clean power source. I've done plenty of wiring in my 61 years, helped out by a now retired electrician.
I can tell you what works to build a street friendly 800 HP engine, but not wiring upgrades/tweaks that have been proven to work. I don't sit and listen to every little detail in any music. My system is in my shop. So it's where I jam out. Just got me some ForteIII's and everything from Buddy Guy to Robin Trower hits hard.

What size breaker does the subpanel use?
50amp  6ga wire
I agree. I will run from main panel