Go into Xonotic, open the cmd window (hold shift and press tab), then type seta "userbind22_release" "toggle chase_active"
You might need to restart the game, I'm not sure.
Then go into the binds menu (in settings) and then find the userbind (scroll to the bottom) and then select a key for it. I've set it so that whenever I press the hash key (#) I go into 3rd person.
Repeat the process with these commands:
seta "userbind24_press" "chase_back 400; chase_up 50"
seta "userbind25_press" "chase_back 100; chase_up 10"
and assign different keyboard buttons for both of them if you want. These commands change the views you get when in 3rd person. The first one is great when you are in vehicles, the second one is better when you're on foot.
Every time you start Xonotic, going into third person will always use a default camera angle so you definitely need to press the binds in order to change the views. I'm not sure how to change the default camera angles for 3rd person...