In terms of ability RVP, Mata and Rooney are the top players.
Importance to the team will vary depending on Louis van Gaal's tactics since a player like Strootman was as key as RVP and Robben in his netherlands side prior to his injury but his ability was not on that level.
For all we know De Gea might show himself to be a world class sweeper keeper, Jones a top ball playing centre back, Januzaj an excellent winger or even Kagawa could excel due to the similarities between his skill set and what I think van Gaal demands. Thus I can't answer who the three most important players will be until I see the tactics.
Liverpool's three players don't worry me, I doubt they worry many opposition fans actually.
Liverpool's best chance seems to be Louis van Gaal making shocking mistakes (very possible) that see United fall behind early although I do think it's more likely he will get it right from the start.