On Magnum P.I., was Higgins actually Robin Masters?

No.
Higgins is a friend of Masters' just like Magnum is.
Masters actually sort of shows up in some episodes, I believe.

He does? My mom was a fanatic for Magnum and said he never showed.

Robin’s Nest is managed by Englishman Jonathan Quayle Higgins III (played by Texan John Hillerman), an ex-British Army Sergeant Major, and is guarded by two highly trained Dobermann pinschers, Zeus & Apollo (AKA “The Lads”). Near the end of the series it was suggested that Higgins and Robin Masters might be the same person. In the final episode Higgins ostensibly admits to being Robin Masters, but at its conclusion he recants, perhaps whimsically telling Magnum that he had lied about Robin Masters, leaving viewers to wonder whether he was lying about Robin or lying about being Robin. During the early seasons of Magnum, the voice of Robin Masters (heard only a few times a season) was played by Orson Welles.

Ah, Orson.

yeah I remembered him having a voice on the series a la Charlie.
though perhaps it is a bit more fun to imagine that Higgins is Robin Masters, etc etc.

What were the names of Magnum's friends again -- Rick and TC, right?

perhaps the big-screen adaptation will answer these questions

I thought that was going to be about the Simon & Simon crossover, which was on earlier this week.