Generally, a 3% solution is by weight. That is 3 grams of solute in 97 grams of liquid. It can also be defined by volume especially when both components are liquids. Then it would be 3 ml of part a and 97 ml of part b. Often it will be described as "measure 3 ml of part a and dilute with part b to 100 ml."

With aqueous (water) solutions it doesn't make much difference since a milliliter of water weighs about 1 gram. Also, the concentration of cleaning solutions isn't super critical. But 'more is better' doesn't always apply either.

BTW, for reference, 1 oz is about 30 grams.

I would assume you need to make enough 3% solution to recirculate in your cooling system. A quart of liquid is about 900 grams. Figuring about 10 quarts in the cooling system, you would need 270 gr (about 9 oz) of cleaner in 10 quarts of water.

You might want to rinse the system a couple of times before adding 50:50 coolant just to be sure no incompatibility between cleaner and the corrosion inhibitors in the coolant.

Good luck.