I've ridden three speeds about 60 miles. I do have them geared down a bit with a 21 or 22 tooth cog in back.

I rode the MS150 (75 miles a day) a few years ago with my Raleigh Sports, but it has an S5 5-speed hub so it was a bit easier. Got a lot of odd looks. The funny thing is several people were very concerned that I would die or something.
I thought I should do it again on my DL-1, wearing knickers, a bow tie, and dress shirt.

I rode a century in 2020 on my touring bike , and it was fine up to about mile 80. Between 80 and 90 miles is when everything started to bother me--chafing, rubbing, soreness--but I was going to finish. I imagine on a 3 speed it could be tough.