Ally's hotel room is on the 15th floor, and she parked her car 3 floors below ground level.
To determine the total number of floors she traveled, we can add the floors below ground to the floors above ground:
- 3 floors below ground level (this is negative when counting upwards)
- 15 floors above ground level.
So, the total number of floors traveled is:
\[ 3 + 15 = 18 \]
Therefore, Ally traveled 18 floors in the elevator to get from her car to her room.