if the rear cluster is a cassette and not a freewheel, an option is to go 3 x 7 on 8 speed spacing. or 3 x 8 on 9 speed spacing. in either case,
you can use your existing wheelset. you simply remove one of the gears from the gear cluster to make it fit on the hub. (7 cogs from 8sdp cassette, and 8 cogs from a 9spd cassette). you would also need new shifters, though. i know...you said yours are working fine, but the question is for how much longer given their age?
mind you, i'm not one towards "upgraditus" wherein more/newer is always better. thing is it's getting harder to retrofit as availability and practicality is dropping off as technology continues to advance.
for example, i went to modernbike to see what would be available to you within your given preferences. darn near everything is currently unavailable. so, unless you're somewhat or fully focused on getting a reproduction of your existing set up, it might be wise to flex a little within what's currently available.
to be more specific, trying to match the 30/40/50 with the same gearing in the rear is going to be a challenge. you can still get the same gear ratios and approximate feel of shifting with a 28/38/48 (as suggested already) and a 12-28 cassette either in an 8 or 9spd cassette. you'd simply get a cassette (ie. 8spd) and drop the 32t cog off the cluster with 12-28 remaining.
https://www.modernbike.com/sunrace-c...speed-cassette
make sense?
here are some 8 speed shifter options
https://www.amazon.com/shimano-8-spe...+speed+shifter
8spd chains are also inexpensive. keep your current front derailleur and get an 8spd rear derailleur in alivio, acera, or even deore (if you want to spring for a little extra). any of those components are quality enough for practical purposes. and, of course, get a crankset as suggested above
hope this helps!
