Hi
Apple Watch Series 3 (GPS + Cellular) is not currently available in South Africa.
Apple advises that Apple Watch Series 3 (GPS + Cellular) will become available in other countries this year, but it has not yet specified when and where. I suggest that you monitor the Apple website for further information regarding potential availability in due course:
Apple Watch Series 3 - Apple (ZA)
Apple also does not currently list any supported carriers for South Africa:
Apple Watch - Carriers - Apple
I'm afraid that, in the meantime, no-one here can tell you anything about Apple's unannounced plans. You are not talking to Apple here - this is a user-based technical support community.