Seriously folks, I really appreciate the creative answers but... no one has answered the question:
what fight is harder: 3 hill giants or 2 frost giants?
2 frost giants, definitely. Hill giants aren't too outrageous for CR 5, while frost giants are tough even for
their CR 8 (I got it wrong earlier). It's not just raw damage (+9/25x4 vs +8/18x6) - IME frost giants are a LOT more durable - better AC 15 and hp 138 vs AC 13 hp 105 is nice, but look at the saving throw line - hill have no good saves and are vulnerable to a clever caster, frost have CON +8 WIS +3 CHA +4 (as well as
STR +6). 2 frost are worth 7800 vs 3 hill 5400 and I'd say that reflects the threat level.
When you fight frost or fire giants you KNOW you're fighting giants!