Larian Studios Forums Divinity - Original Sin Divinity - Original Sin - General 3 skill trees, too much?

Not having enough ranks in a skill tree for a particular spell only means that you have to pay an extra 2 AP for each rank you are missing.
There is only 1 spell per mage tree that requires rank 5, so if you are OK paying 2 extra AP for those you can get 3 trees to level 4 without investing additional points.
My recommendation is to bring all 5 trees to level 1-2 first instead of focusing at leveling 1 as fast as possible.