Two blocks of unequal masses are tied by a string. The blocks are pulled stretching the spring slightly and the system is released on a frictionless horizontal platform. Are the forces due to spring on the two blocks equal and opposite? If yes, is it an example of Newton’s third law?

Yes the two forces are equal and opposite. But it is not an example of Newton's third law of motion.

As in that case, the forces should act on the two objects which exert force on each other.

The action reaction pair would be the force acting on the spring and the force exerted by the spring on the mass on th both sides.

There would be thus two action reaction pairs in this case.

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