In the bridge simulator at the start of Star Trek II pitched a refit against three K-Tinga's and the whole point of the mission was that the ship was doomed. So I'm gonna say 3:1 is pretty poor odds...
Sheet 6, of 14, of the Star Trek: The Motion Picture Blueprints
says that the Enterprise
(upgrade) class promises to even the odds for one Federation starship versus three "Drell-4" (K't'inga) Klingon heavy battle cruisers.
I see three main possibilities:
1. The hope that the odds for 3:1 would be even was optimistic.
2. The Klingons didn't stand still when the Connie refits started rolling out. If the odds for 3:1 really were even when the upgrades first came out, then only relatively minor upgrades on individual K't'inga's would have tipped the balance back in their favor, especially when patrolling in packs of three or more.
3. The Kobayashi Maru scenario invariably depicted the worst-case performance for the Federation starship.
Any, some, or all of these could be true.