Hi
I'm afraid that no-one here can tell you anything about Apple's or Virgin Mobile's unannounced plans. (You are not talking to Apple here - this is a user-based technical support community).
If you would prefer to be able to use the built-in cellular feature on Apple Watch Series 3 (GPS + Cellular) in conjunction with a service plan for your iPhone from Virgin Mobile, then I suggest that you contact your carrier to let them know and to ask about their plans for supporting the watch.
You may otherwise wish to monitor this page for new carriers being added over time: