Get a third key?
 

I want to get a third spare key for my sister in case she needs to drive it. I want to know how much usually the spare key cost and how long does it take to order one from the dealership.

Don't know what vehicle u have but the dealership
Should stock the key so no ordering. But if its a newer mazda it needs to b programmed. Programming is usually around $50. The key depending on what car u have can be anywhere from $30 to $300.

I had a regular key cut for mine. I use it as a back up to lock the car while it's running, or carry it with me in case car gets locked somehow with keys in it (this may be hard to do, i wonder if you can lock the car after you open door by pushing the lock button on door, i normally use the key).
However, the key that i had cut, won't start the car. It is just a key, not any push button thingy's. Take racers advice as he is a mechanic and he works for Mazda.