Answer: Tre wants to mow 20 lawns in 5 days, so he needs to mow 20/5=<<20/5=4>>4 lawns per day.
If Tre can mow 3 lawns in 2 hours, then he can mow 1 lawn in 2/3=<<2/3=0.67>>0.67 hours.
This means Tre will have to work 4/0.67=<<4/0.67=6>>6 hours each day to mow all 20 lawns. Answer: \boxed{6}.
Explanation: