On form and under Roy's system of play, I'd say probably those three too, though Mulumbu did have a questionable start to the season.
Still, players often drop in and out of favour. Long has had a very good start, no doubt, but in the grand scheme of things it's still early days. I hope he stays good, but he wouldn't be the first striker to hit the ground running then fall away for part of the season. Roy himself has stated that it may well get much harder for him. Look at Pete, he provided a great assist and goal return last season, now he's seen by several as dispensable.
In terms of monatary value:
Isn't Pete our highest paid player? I guess in the summer he was seen as our most valuable asset in some ways by rewarding him higher pay than anyone else in the squad.
With age on his side, strikers at a premium, I'd guess that Long would probably fetch the biggest transfer fee.