Hi
Apple Watch Series 3 (GPS + Cellular) requires an iPhone 6 or newer model, running the latest version of iOS. It is not locked to any specific carrier and has a built-in eSIM that copies information from the iPhone's SIM card.
The built-in cellular features require a carrier plan for your iPhone that specifically includes support for Apple Watch, including the carrier activating the device on the plan.
Simple Mobile is not currently listed by Apple as a supported carrier for Apple Watch Series 3 (GPS + Cellular):
Apple Watch - Carriers - Apple
I suggest that you contact them to ask about their plans for supporting the watch on their network.