C wire on a multi taco zone valve boiler

looking for assurance on what I think is the right setup to add a c wire to a current 2 wire zone setup. See pic of transformer junction box.

Currently I have 3 zones using one transformer and 3 taco zones. All stats use 2 wires, Rh and w.

Looking at my transformer setup it looks like they took all the stat wires from the different apartments, zones, bundled them up and connected all the Rh wires pigtailed together to one of the leads on the transformer. ( see pics. Note I didn't remove junction cover yet )

I'm assuming they then pigtailed all the W wires from the stats along with all the wires from taco zone terminal 1 together and pigtailed together to the other transformer lead. Then took taco zone terminals 2/3 pigtailed out of the junction box (I saw the 2 wire cable go to relay box) to relay then boiler on the t terminals.

My doubt is if I introduce a new stat wire for c, where do I connect the c wire. Should that go on the bundle of wires that connect the stat W and zone valve terminal 1?? Or is there a better way.

Also the transformer terminalit doesn't seem to be marked with + or – so am assuming you can just connect the R/W to any terminal as long as it's consistent.[![enter image description here nhi][2]

Thanks very much for the forum and everyone that's been helping.

Paul

[2]: https://i.stack.imgur.com/M7vrA.jpg