SA 3 speeds ARE 100% wonderful, I know because I do hundred mile rides on mine at 50 lbs with drinks/ coat. I service/ take apart mine once a year, it was new in 2017 and is a drum brake version. Grease on the bearings and a couple spoons of oil inside. My GIs are about 49 to 87. You can easily lower that to 40 to 69 with 44/23T cogs. My bike is 100% silent, only somebody riding behind might hear the ticking. NO hub is quieter.

However your ideas are 99% harebrained. You can't just take any derailleur bike and switch. Get a bike old or new that already is a 3 speed with a 120 mm dropout. Derailleurs were never easy IME.
I also have a Nexus 7i that is pretty good, but has a roller brake that drags like hell.