I had three ganders bond with a goose last year and it wasn't good. I removed two of them. Poor goose had a bald neck very quickly.
I'm not sure if you can get them to pick a new mate. The grabbing sounds like attempts at mating or dominating. Personally I'd remove one, or two, but I know not...
No expert on these breeds, but the three African all seen to have long necks and are protective of the white one, so I'd guess you have 3 African gander and a Chinese goose. Definitely wait for more opinions than mine.