Is the Everglades worth using one of only 3 days in Miami?

I think so, but I'd do a half-day trip rather than sacrificing the entire day.

Leave early, do an airboat or wildlife-focused Everglades experience in the morning, then come back to Miami for the afternoon. You can still fit in Coconut Grove, Vizcaya or Key Biscayne afterward.

Just check what you're actually booking because some "Everglades tours" visit wetland areas outside Everglades National Park rather than the park itself.