Is the buoyant force on an object is the reaction force for the force exerted by the objects on the fluid (newtons third law)? Or there are two sets of action reaction force? (objects force and reaction , buoyant force and reaction)
Suppose you have a weightless box that is 1cm square.
Suppose you push it down into some water, to a depth of 1cm.
Since you are displacing 1 cc of water, the weight of the water you are displacing is 1 gm.
So you are pushing down on the water with a force of 1 gm (I know that's mass, but bear with me).
The water is pushing up against you with a force of 1 gm. That's the reaction force against your push. That's also the bouyancy force.