How much is it for 3, 50pound suitcase ?

Depends on the airline. You can check on their website.

Each airline sets their own prices.

You need to call your airline and find out some each airline charges a different price. The airport itself can not tell you that because that are not responsible for the airlines prices

Depends on the airline

Depends on the airline but generally its 25$ per suitcase

Varies by airline.