Unless the Watch3 is extremely popular, Koodo won't do it. The problems are with the sharing plan and the implementation of an electronic SIM on a budget carrier. Rogers even said they "look into it" without any promise.
Koodo doesn't offer tablet or data-only plans. All Koodo plans are talk & text, with or without data. The only device Koodo offers that isn't a conventional cellphone is the home phone adapter (to compete with most telephone companies other than TELUS).
Koodo does sell accessories (phone cases, etc.). Smart watches that connect to your phone via Bluetooth/Wifi can be such accessories. Smart watches that have a sim reader and connect to the cell network fall under Internet of things and alternative cellular devices, which Koodo won't do.
You can get an Apple Watch 3, pair it with an iPhone that you got from Koodo via Bluetooth or Wifi and just use it as a smart watch, but no dice on getting the smart watch as an add-on to your post-paid subscriber line or anyway of getting the same number deal.
With all that said, my bill will go up by about $40 per month, but I will save a few bucks on my Bell TV service as part of a bundle. I'm sorry to have to leave Koodo, they have been very good and I am keeping the rest of the family on there. As I earlier said, I'll consider switching back if and when they ever support the eSIM / Apple Watch.