Question

how Newton's third law is related with the movement of an object a constant speed in space

Answer 1

When an object is moving around Earth or Sun or another planet, Newton's law is applicable. The gravitational force is the action and reaction force between them. Still the object moves with a constant momentum in a circular path.

If an object in free space very far from planets and stars, then it moves at constant speed (momentum) . There is no force on the body. So there are no action or reaction force. So third law of Newton is applicable. But forces are zero.