The rulebook states that you can use one action per 100pts of your teams build per turn

300pt build = 3 actions per turn

If I were to playing a 300pt game and my teams point value is 299 do I still get 3 actions per turn or do I only get 2 actions per turn. Basically, do I have to hit 300 pt exactly to get 3 actions per turn in a 300 pt game?

The number of actions you get is a function of the build total of the game, not the size of your force. You could, theoretically, just take 100 points of figures in a 300 point game and you'd still have three actions.

It is based on the build total for the game, not so much the particular build total of your force. So if it is a 300 point game, no matter your team build -- which cannot exceed 300 -- you would get 3 actions.