The whole system takes a lot more than 3 gallons. It's more like 5 or 6. Your owner's manual should have the cooling system capacity, listed in quarts. You will want to buy half the system's total capacity for a 50/50 mix, which is what's recommended.
Preferred method of home flushing:
If your heater hoses are accessible, get a Prestone flushing tee kit and install/use as per instructions. Flush till nothing but clear water is coming out. Turn off the hose, remove it from the tee, and open the radiator drain. Close the drain when water stops running, leave the cap off the tee. Fill the radiator with the calculated amount of straight coolant, you will see plain water run out of the tee. When all the coolant is in, cap the tee. Then you can run the engine and burp the air, topping off with water as necessary.
If the heater hoses are not accessible, drain the radiator and try to get the block drain plugs open. Replug and fill with water, run for 15 minutes, repeat till clear water is all that's left. With both the radiator and block drained, you can easily get half the system capacity's worth of coolant in there. Without draining the block, you probably can't.
If you don't have accessible heater hoses and you can't get at the block drains, I'd consider having a garage flush it for you with a machine.
If you are going to use Prestone coolant, *ALL* the factory stuff has to be flushed out. Not the same stuff as the factory "long life" coolant.