Choice in inch size tires is getting very limited.
Metric sizes are usually tubeless, not a big problem but mounting is more difficult due to the tubeless bead profile: even seating is not easy on a tube rim.
3" is a 75mm wide metric tire. Good luck finding that!
90mm is 3.5" and these are not so difficult. This will fit nicely on the front or rear. A 100mm tire will work on the rear. It was common to use the 4.10 K81 and it fits on the rear. I'm not recommending K81 tires now, the 4.10 is the only size possible for a 400 and it's too big for the front (they will go on but only deflated to pass the fender hoop bolts) plus 4.10 (actually 4.21) wide tires on the WM-1 1.6" front rim is way oversize and still over on the WM-2 1.85" rear (2.15" WM-3 rim is listed as minimum size).
In the end pretty much any new tire will be better than aged out and hardened tires.
How do you ride it? Trying to keep up with 600 sportbikes in the twisties? ... forget it and just get one of them. Aggressive riding testing the limits of the chassis and tires? you want the best tires you can find (also most expensive and shortest life).
Since you have "senior citizen" tires that aren't worn out, I think that most any tires will be fine for you. The brands you haven't heard of like Shinko, Riken, Duro, etc. are not bad tires. They tend to have bought old moulds from other companies when they discontinued these unpopular sizes. I have used the Duro Dunlop K77 clones with no problems, for example.
- don't go way oversize, check the rim compatibility for tire sizes over stock
- get tires that suit your riding style. Even "el cheapo" tires will be fine for casual riding - you will be slower at the limits but do you really want to test limits on a 45 year old bike?
- use new rim bands and tubes, try to get name brand (Pirelli, Michelin) tubes.
- rims will probably be rusty inside, clean with a wire wheel and then paint with rustoleum or similar.
- check spoke tension while the tire is off. Tap each, the ringing sound should be about the same pitch for all. Tighten thunkers if possible. A couple of thunkers is not great but probably OK, respoke the wheel if you find serious tension ineqaulities.