The adapters are to convert the "Triumph mounting method" into the standard used by other mirrors for Harleys and the like, like Kuryakyn Magnums. Doubtful that you need those for the Triumph mirrors. Might try to hunt down some rubber O-rings and normal washers and cobble something together, I guess?
I've already decided on the Magnums, I like the massive look of them for the R3T and I want big glass, the round ones that come stock are just garbage. Probably no coincidence since Triumph has multiple optional models for sale...