You can't include Africa for a three continent Oneworld Explorer, it will at least be a four continent.
Mauritius is a pain to include in a OWE at this point, and you cant do it without an open jaw.
My suggestion is to do a 3-continent Circle Indian, and go something like BOM-HKG-NRT-LHR-DXB-LHR-MRU//JNB-HKG-BOM. The segment between Mauritius and Jo'burg is an open jaw and will run you about $300, if I remember correctly. I bought my ticket ahead of time, since it was imperitive that I was able to get on the flight, but I think if you have time to wait and a little flexibility, you could probably get that segment a bit cheaper in MRU.