An object at rest tends to stay at rest, and an object in motion tends to stay in motion, unless acted on by an external force.
Newton's first law is commonly seen as the phenomena known as inertia1). It applies in all cases where the forces balance and the object is not accelerating. Mathematically defined as $F_{net} = ma$ where $F_{net} = 0$.
An object in motion tends to stay in motion.
In an inertial frame of reference, the vector sum of the forces $F_{net}$ on an object is equal to the mass $m$ of that object multiplied by the acceleration a of the object. Mathmatically the same as the first law: $F_{net} = ma$2), but where $F_{net} \ne 0$.
Every action has an equal and opposite reaction.
The third law states that whenever one body exerts a force on a second body, the second object exerts a force that is equal in magnitude and opposite in direction. $F_a = -F_b$. Note that it is the force that is equivalent. With the 2nd law, this means that they can have differing accelerations after the application of the force depending on mass.