Goin to 25 same time atm with all ur characters can b done fast. To 26 it takes bit more time.

It all dpd on u, its always good to have a choice when u joinin a team you can maximize the efficiency. (Class skills or class need for class spec room)
Its also good to play all 3 if u lack experience, so u understand better the game or class interactions in general.
Might be expensive on the long runs with higher lvls comin or better pink color gears.

Or

You can just play and focus on only 1 class atm. Its less time consummin, cheaper on the long run as you farm only for 1 class.
And u can always create other class later when u feel ready or have excess cash/gears.