I'll have to come clean here Benjiesdad.
I won the Manx 2 day in 1976, and the Senior MGP in 1983. (plus TT in '93)
I DID ride the 2day AND the MGP in the same week, in 1976, 80, and 81, best result was 4th in the 2Day, and 3rd and 4th in the 2 races, ie Junior and Senior, in 1980.

Back to the quiz question, it certainly won't be a long list of riders doing all three disciplines, and I'm already surprised that there are three, namely, Richard Crossley, Mr Hawes and yourself.
I thought I'd done well listing Florian Camathias as a solo and sidecar competitor, did anyone else get him? He was crazy enough to have passengered aswell!

I'll keep researching.