I live in a condo and my toilet seems to be the source of a leak to the unit below mine. I was told that it is the wax seal that is faulty. The bathroom was remodeled 3 years ago. The wax seal should last longer than 3 years. Should the contractor that installed the toilet be responsible for the repairs?

No. Forget that concept entirely. The wax ring seals against sewer gasses. It will seal against water leakage only as long as there is no pressure. If you have a blocked sewer pipe it will sometimes hold. But such a simple thing as plunging will break the seal.
Best thing you can do is when the toilet is remove to replace the wax ring, seal between the flange and the floor with caulking.
Do not caulk around the toilet and the floor. That way, if there is water under the toilet, it will run out where you can see it and take corrective action.

Remember its only a glob of wax.

hkstroud is absolutely correct.

Agreed, and sometimes a seal will need to be replaced, as noted it is just a piece of wax. If the seal is bad, after three years, no, the contractor would not be responsible

Does the toilet rock or give a bit when you set down? The leak and repair is on you and not the plumber. Good luck, Tom

I have a feeling the toilet was not set properly after the remodel - especially when floors were redone and new floor tile installed. There is a chance one wax ring was not thick enough therefore not reaching the bottom of the toilet and not making a positive seal. During flush, turbulence of water will spray over the wax ring and will follow the pipe into the ceiling.

Second scenario may be that who ever set the toilet, mis-set the toilet and in the process squashed the wax ring to one side. As consequence, every time toilet is flushed, a little bit of water gets pass the wax ring and into the ceiling.

Deterioration of wax ring in three year period is a very distant possibility.

I suggest you lift the toilet and inspect the ring.

Hope that helps

Milo

Toilet bowl gaskets have not been made of wax for a long time. A correctly installed bowl gasket will not get wet unless there is a back up in the piping connected to the toilet flange, and, a correctly installed gasket can withstand the very slight pressure imposed upon it by an overflowing bowl.
Both the IRC & the IPC require caulking, or grouting all plumbing fixtures where they come in contact with the floor. A water tight seal at the point of fixture floor contact helps to maintain sanitary conditions in the event of a soiled water overflow, or, fecal / urine spillage...