Longest day I've done on a 3-speed is 50-60 miles. Terrain, of course, will determine whether you're wishing for more gears. I've thought I might try this one at some point on a 3- or 4-speed:

https://ridewithgps.com/routes/856676

That's 100 miles with about 4K feet of climbing, so "flat" is definitely relative in this part of the US!