Rarely is a three way timer switch (if such an animal exists) useful. But a regular timer switch can override a 3 way switch setup to do one of: hold the light on all the time or keep the light off all the time.
When the timer is overriding the 3 way switches, you cannot easily set the 3 way switches so the light does what you want when the timer stops overriding, except if you remember which 3 way swithc positions do what.
Do you get coverage you can count on? From the bag of fertilizer? From your lawn sprinklers?