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

@jea48 .
The OP says his line comes from a subpanel that also has the AC compressor.
Would it make sense to simply add another subpanel from the main?
Would that isolate the effects from the AC unit?
(OP-sorry if I am making this more expensive. I only want to see if there is a way to keep the AC from affecting your stereo).
Bob

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.

I’m not @jea48, but until he chimes in, I can tell you that I run a compressor as part of my hi-fi system-- fairly substantial, at one point a 1 hp 13 gallon unit- but even with smaller units, there was a nasty electrical snap that could be heard through the system when the thing cycled. The way I resolved that at the time was to run it from a 240 line with a step down transformer.
When I moved, I had more flexibility and installed a dedicated line just for the compressor that went nowhere near the subsystem feeding the hi-fi. The hi-fi has it’s own iso tranformer (10kVa- a beast) that runs to a sub panel that feeds 10 gauge Romex to the outlets. The compressor is not coming off that big iso transformer but comes from the main panel for the house and has a smaller iso transformer as a buffer.
You are probably creating a little bit of racket in a hot rod shop, but it’s worth checking to see how much noise you hear through the system when things are quiet. That "snap" was quite pronounced for listening purposes in a quiet, dedicated room. Dunno if that helps. Love to hear about the hot rods though!