Hi and thank you for your question.
I am guessing you do mean a connecting rod because I have not actually ever heard anyone use the term piston rod before.
Replacing one connecting rod on a triple crank is not something most home mechanics and professionals would attempt. A much more viable alternative would be to replace the crankshaft as an assembly. The main problem with home mechanics is they don't have a hydraulic press to separate the cranks halves to install a new rod nor the truing fixture and expertise to true the crank if they actually got it apart and back together. Single crank rebuilds aren't too bad, twins are a headache, triples are a nightmare for the novice. Just too many moving parts and too easy to get it wrong.
You have a couple of options on buying a crankshaft. You can go new factory Yamaha part for about $1400 or go with a rebuilt crank for around $400. Understand, you get what you pay for, but it may not make good financial sense to drop a $1400 crank into a ski that is probably only worth $1500.
If you are going to do it, invest in a service manual. It will help guide you through the process, although understand service manuals are not intended to be text books to train people how to work on skis. It is a reference material written for a professional technician with 2 years experience and assuming many things are understood and not mentioned or addressed in the service manual.
if you dont want to tear your engine down, there are rebuilt short blocks that you can get where you pull your engine out, strip off the intake, exhaust and ignition from your engine and put it on theirs, then send your core back to them.
These rebuilt engines and cranks will not give the same service life as OEM parts, but then again they don't cost as much. If you want to get a couple of years out of the ski, they should be fine for that. I would nor recommend going with rebuilt parts if you intend to keep the ski for a very long time. You will end up replacing the parts again.
The only other tip I will give you is no matter which direction you go on the repairs, you will get the absolute longest life out of whatever you end up with if you use the Yamalube 2-W watercraft oil. There is a difference and it will save you money in the long run. If you go cheap here, it is going to cost you big time in the long run.
If you have any more questions, feel free to ask.