Your opponent is also utilising the three dimensions. They have no choice in the matter.

If they swing their right shoulder, rotate their waist, shift their weight into the right foot - in an attempt to hit your jaw - they are employing two dimensions.
They are using the horizontal axis, and moving forwards.

Your response must be applied relative to these dimensions.

If you attempt to oppose the incoming force by using an outward-moving 'block' with your left arm, you are going to meet the full force of their delivery.
This is not tai chi.