Timer with a Dimmer Switch
Can I add a dimmer switch to adjust the brightness? 


Yes, the dimmer would just be added in series with the timer.


  And change the timer switch to a three-way switch


Normally, a timer controller goes on the feed to the circuit for outside lights without any other switches. However, you can wire things any way you have the imagination for within reason. If you connect the timer as a 3-way the other switch will override the timer anytime it is operated. If that is how you want the circuit to operate then it can be done.

A switch like this:
https://www.amazon.com/Honeywell-PLS750C1000-Ti072-3W-Sunrise-Required/dp/B01IAEPFD4
Can be used as a single pole or 3-way switch.

Good luck and stay safe!