Why are there only 3 Newton's laws of motion?

NEWTON’S THIRD LAW OF MOTION states that for EVERY ACTION, there is an EQUAL and OPPOSITE REACTION. The FORCES of an ACTION and REACTION between bodies in contact have the same magnitude, same line of action, and opposite sense, or in other terms, they are equal, opposite, and collinear. read more

There was a fashion to state everything in three laws (Kepler had done it before him) and he followed suit. Actually the laws of motion are just one law. Mathematically, F=ma (Force=Mass into acceleration) covers all the three laws plus laws of inertia, law of conservation of momentum. read more

Newton's First Law of Motion, or the Law of Inertia, states that "every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it." This means that there is a natural tendency for a moving object to keep moving, unless an external force interferes. read more