If his anger doubles every 20 minutes, then, because there are 9 lots of 20 minutes in 3 hours his anger will have increased by 29 or 512 times!
I'll expand on the above a little:
After 20 mins (1*20mins) his anger doubles, i.e. increases by 21
After 40 mins (2*20mins) his anger has doubled again, total increase = 21*21 = 22
After 60 mins (3*20mins) his anger has doubled again, total increase = 22*21 = 23
After 1 hour 20 mins (4*20mins) ... , total increase = 23*21 = 24
... etc.
After 3 hours (9*20mins) ..., total increase = 28*21 = 29.