The answer is "it depends". Mainly it depends on how heavily you want it applied and thus how fast you drive. I put down 26 gal of Roundup on a plot probably between 3/4 acre and 1 acre. A few weeks later, I drove the same area and killed what had come up new, using about 12 gallons that time - so I drove faster.
It doesn't come with instructions on how heavily to apply things. You will need to get that info from the packaging on whatever product you are using. The product mfg will tell you how heavily it should be applied.
With 26 gal and 2.2 gpm, figure you have about 10 minutes to empty the tank and so drive accordingly. So for Roundup, my guess is you'd need 3 tanks to do 3 acres. If in doubt, I'd suggest driving faster, then go over the same area again if you covered the area and still have material left in the tank.
You could always start by filling it with water and try a test run to see how much ground you cover at a certain speed before it runs out.
You could mount it on most anything, with a little ingenuity and making some brackets, etc
