I've gone up hills in 3rd gear on my 3-speed on occasion that normally I'd use 2nd or 1st if I'm tired. You have to stay 'on top' of the gear, which basically means going like hell into the hill and cranking as hard as you can to stay at a high enough cadence. Still, when I'm tired at the end of a ride, loaded up with lots of stuff, I'm glad to have a couple of lower gears.

I think an IGH setup has many of the advantages of SS/FG (no derailleur to get bashed or clogged up with mud/snow, single fixed loop of chain etc) but gives you more options at the expense of a bit more complexity. Apart from the 3-speeds, they're also difficult to repair. For flat areas a single speed is perfect.