So I have 3 deciduous trees.. What is the best n-p-k fertilizer I could use? Or should I go organic?

Pellets or liquid?

I understand each one is different according to the area you wish to develop further.

I've also been told by the local nusery person that phosphorus and potassium help the growth of roots thus making the tree/plant thrive in its environment. This person recommend a high balanced liquid fertilizer such as 10-10-10.

Would that be ideal here?
My trees are about 1 and a half year old. If that changes anything as far as fertilizing goes.