3 wire stat on old setup

Hilly

I got asked to go out of town to upgrade some thermostats.
It is a 3 zone valve system, with an electric boiler, and traditional digital only thermostats.

When asking questions.
The system seems to have 18-4 ran so there are 2 unused wires available.
It is just wired with a 120v/24v 50VA Transformer.
If you just have a 24V transformer where would you wire the C in this setup?
Also can I put 3 nests on one 50VA Transformer? I thought I read hotrod say once there was a recommended limit on some of this wifi stats. I'm not home yet and search some of this stuff on my phone isn't the easiest. So anyone that may know off the top of their head I do appreciate your response and stored knowledge.

I just don't want to go help somebody out and not take all the material I need. I honestly stopped wiring without the use of zone valve controls and relays panels a few years ago. So I don't think I ever wired a power hungry stat with a C wire that didn't go in a panel.

GW

Unless it's you're own house why do you "not" just install a zone control board? Anyway, seems like you're good if you run a c from the trans to the c on your stats

I've seen Tacos have a problem with "sharing" the c with the heating system. So, don't bet your dog on this one

Hilly

Thanks GW but in this case it's my time but not my money to spend with regards to a ZVC and everything is special order where I come from so I don't have that option. I just wasn't sure if it was okay to just directly feed the C of off my 24V hot (R) leg of the transformer. I guess I asked for 2 reasons. I wanted to know if most guys would use a 2nd transformer for these hungry stats or would you just do all the math a size it as per the math.