Campag and Shimano do use different splines. Sometimes you can interchange freehubs on things like Zipp wheels.
Also, Campag 8 speed is different to 9/10 for cassette splines. SO you couldn't put a Campag 9/10spd cassette on an 8 speed freehub. Shimano 9/10spd cassettes will fit on the same freehub as the overall width is the same.
The spacing between Shimano and Campag sprockets is different so generally you can't mix them. There is some cross-compatibility but personally I wouldn't bother.
Chainrings tend to be thinner for 10spd than 9/8spd but you usually have enough clearance to make it work.
Don't forget it's the shifters that are indexed for the number of gears – the rear mech just moves across a fixed distance between two points.
You can still source Campag 8 speed cassettes (or Miche ones) but your choice of sizes is limited. Any 8spd chain will do. KMC is fine, in fact KMC 9 or 10spd do the job on Campag and Shimano fine.
Road drivetrains tend to last 3x the miles of MTB as a ruleof thumb.
I'm guessing your 2×8 is a Campag and the 3×9 a Shimano?
I'd go for the big S every time. If you buy the Campag and then start to want little spare parts, or need to service hubs or whatever then you'll understand why 😉