60 to 70 is pretty fast.
They changed the experience needed per level to 70% of what it was.

You can ding 70th, in Outland in Nagrand (Hellfire, Zangarmarsh, Terrokar, Nagrand).
Not even getting into Blade's Edge Mountains, Netherstorm or Shadow Moon Valley.

I would imagine a weekend, if you just did wow the whole time.

70 to 80 will take a bit longer. There is no reduced experience.
With a 3x group, current level instances won't be an option, unless you find a tank and healer.

But you could do a lot of pvp, and every quest should be doable.
It's just collections are more then a little painful.
Still, collecting x3 now, is no worse then x1 at a time, on 3 runs to do stuff.
And you get the kills x1 now, instead of needing to do them on each toon in turn.
So even with collections, it should be a lot faster then 1-boxing three toons in turn.