There is a hompage https://willmyphonework.net/, that unfortunately does not yet list the Fairphone 3/3+.
So, I checked for the Fairphone 2, getting the following result:
- 2G - 900 MHz / 3G - 2100 MHz / 4G - Band 3 - 1800 MHz (all are working)
The same goes for MTN, Cell C and Me&You Mobile
All frequencies are supported by the FP3+ as well.
Here is the result for all 7 carriers, that are listed on the page:
Please be aware, that this is the result for the Fairphone 2.
The Fairphone 3+ might be compatible with FNB and Telkom as well, as it supports more bands.
I would conclude, that it works; though I will not offer any warranty.
You could check with your carrier.
The tech-specs are available on the product page for the FP3+ under -> Network