From what I remember, MK3 didn't last long in the Arcades in its original form. Once the UMK 3 revision came out, literally every single machine MK 3 machine was converted with a new Marquee and new Sideart to go with the update. I assume this would be why a dedicated MK3 machine would be very scarce. The old Marquees and side arts were probably destroyed as well, so they would be hard to find.
Now my question is, was UMK3 an optional upgrade that vendors had to purchase, or was it a mandatory upgrade that Midway provided for free? I find it a bit odd that after the UMK3 update came out, every single previous MK3 machine that I saw was upgraded, especially if it would have cost vendors money to do so.