How do you wire a timer switch to a 3 way light switch?

First you need to decide what you are trying to accomplish. You can wire the Timer switch in series with or parallel to the 3-way switches. If in series then the 3-way switch function would depend on the timer state. If the Timer switch was ON then the 3-way switch could turn the connected device ON or OFF from either location. If the Timer switch were OFF the device would be OFF no matter what you did with 3-way switches. To wire in this fashion put the timer between the supply and the input to the switches.

If the Timer is in parallel then if the Timer is ON it doesn't matter what you do with the 3-way switch, the device will be ON as long as the Timer is ON. If the Timer is OFF then the 3-way switches will work in their normal fashion. To wire in parallel connect the supply to one side of the Timer and the device to the other.