Doesn't your exhaust already have a 'reducer' (increaser) to go from the standard Cat diameter of 60mm to the Milltek's 69.85mm?
They clearly supply adaptors where required...so...
I'm guessing their 3" DP is designed to fit the 76.2mm Race system with no adaptor, or the 70mm link pipe they supply with the cat back, without the use of a further supplied reducer.
I'd be amazed if they didn't supply the adaptor.