I have 3 guitars. An Epiphone SG, Ibanez RG, and an Arias Pro II strat remodel. My amps are a Marshal MG 50 and a Roland 20x Cube. I want to know which guitar fits better for Drop D, Drop C#, and Drop C. The music I intend to play is mostly oldschool hardcore/metalcore like 7 Angels 7 Plagues and Poison The Well.
It depends on the scale length really. Lower tunings benefit from longer scale lengths and thicker gauge strings. No guitar is generally "fit for a tuning".

Get each one set up for a tuning of your choice with strings that suit that tuning.
Main Rig
Korg Pitchblack > Digitech Bad Monkey
Peavey 6505+ 112 w/ 2x12 (Celestion Vintage 30 and WGS Veteran 30)
[Effects Loop] Boss GE-7 > Boss CE-5 > TC Electronic Flashback
The RG would keep tension on lower tunings better, but the difference isn't really that big. You can do it with all three with the right strings.