The capacity of the first tank is 7000 gallons, and if it is filled up to 3/4 full, it carries 3/4*7000 = 5250 gallons.
When the second tank is filled up to 4/5 of its capacity, it carries 4/5*5000 = <<4/5*5000=4000>>4000 gallons.
The total amount of water in the first two tanks is 5250+4000 = <<5250+4000=9250>>9250 gallons.
If Mr. Finnegan fills the third tank with water up to half its capacity, the tank fills up with 1/2*3000 = <<1500=1500>>1500 gallons.
In total, the three tanks have 9350+1500 = <<9350+1500=10850>>10850 gallons of water.
#### 10850