Hi
If you do not wish to sign up a supported carrier at present, then you cannot currently use the built-in cellular feature with another network.
However, Apple Watch Series 3 (GPS + Cellular) can still be paired, set up and used for other features whilst skipping the steps to activate cellular. The cellular feature can then be enabled later, if and when you are using a compatible service plan on your iPhone.
You may wish to contact your preferred carrier to enquire about their plans for providing service plans for iPhone that support Apple Watch Series 3 (GPS + Cellular).
You may also wish to monitor this page for further carriers being added over time: