Hey there! The Apple Watch series 3 on T-Mobile requires a special kind of wearable plan called our "DIGITS Apple Watch" plan. This plan is essentially the same as our Data with paired DIGITS plan which allows the line to roam in North America (U.S., Mexico, and Canada) and all calls and text are billed at the same rate as part of your primary number. You can also check our roaming coverage for Mexico here International Plans | Traveling Abroad without Roaming Fees | T-Mobile
Personally I would not but the watch as the cellular antenna inside has proven to not be as good as the one inside the Iphones. Iphones don't even have a great reputation for wireless tech so its not saying much. I would wait one more year or just go for a wifi variant of the device.