It's not at all bizarre for Labour MPs, or anyone else, to support RW3.
We need to remember that the people it negatively impacts are a relatively small number of people who live in West London, plus Schipol airport & etc. The people it benefits are pretty much the whole of the rest of the country and, in many respects, also the people who live in West London.
Having witnessed decades of economy harming paralysis one might be easily mislead into thinking that there was a deeper reason for it. There isn't. It's little more than Not Above My Back Yard-ism, with only a tiny touch of additional Build Absolutely Nothing Anywhere Near Anything-ism.
As for the potential users... well... it'd be full the day it opened, wouldn't it?