There are two Newton's Third Law pairs of forces here:
Pull of (whole of) Earth on object = – Pull of object on (whole of) Earth
Contact force of floor on object = – Contact force of object on floor
So Newton's Third Law gives no relationship between the (gravitational) pull of the Earth on the object (call it W) and the upward push (call it F) of the floor on the object.
Indeed, W and F aren't necessarily equal and opposite. For example, if the object falls on to the floor from a height, there will be a short period during the impact when F is of greater magnitude than W.
However, after a short time, F will become equal and opposite to W. We know this because the object will remain at rest on the floor, and therefore has no resultant force on it.
You may not wish to read what follows. It goes more deeply into what's happening, but it's not usually explained in textbooks, and you may find it too weird...
How can it possibly come about that F is equal and opposite to W? Remember that it does NOT follow from Newton's Third Law. What's even stranger is that an object of twice the mass (and therefore acted on by a W of twice the magnitude) must also experience an equal and opposite F on it – unless the floor gives way! How does the floor manage to supply a force that is always equal and opposite to W? The answer has already been hinted at: "unless the floor gives way" wasn't entirely facetious… Even if the floor doesn't actually give way it deforms a little. The heavier the object the more the floor deforms, and the object moves downwards a little. The more the floor is deformed the greater the upward force it exerts on the object (just as a spring exerts a greater force the more it is extended). The object achieves equilibrium when the floor is deformed enough for F to be equal and opposite to W. [You probably won't see the floor deforming; we hope the floorboards are thick enough not to deform very much, but there are methods (e.g involving interference of light) which will show the deformation clearly.]