Read the law well!
If body A exerts a force on body B, then body B exerts a force (equal in magnitude but opposite direction) on body A.
So: force on B from A = - force on A from B.
We have two forces on two different bodies.
So your conclusion may be: If the earth pulls on us, we pull on the earth (via gravity! Not via "pushing with out feets". Like the sun on the earth and vice versa). No sand at all so far. Well, we don't fall but the sand $brakes^1$ our motion, so it certainly exerts a force on us but less than the earth does on us! No one ever said, that the force of the earth on us is the same as the force of the sand on us (remember: the law speaks of two forces on two different bodies).
On the other hand, it is of course true that we exert the same force as the sand exerts on us on the sand. That's why the sand moves away.
1: This is not the correct physical formulation; constant speed, no force, blabla. Of course, but the point of this answer is to make the distinction between on whom the forces act. And not on an exact description of a non-linear motion.