Typically you place bio balls in a chamber like a waterfall box. Most pond filters have a section to prefilter the water before it flows to the bio ball section. The prefilter is typically a filter pad to remove debris from the water keeping the bio balls from getting clogged up. In a pond filter the bio balls over time will get coated in "good" bacteria or nitrifying bacteria which breaks down fish waste, uneaten food and other organic material keeping toxic waste levels safe for your fish. If your pond has a overflow box than the bio balls can usually be incorporated Can you use too much?

How much would be needed for 3 gals of water during water changing for my 10 gal tank?

We recommend using 1 teaspoon to treat 10 gallons. To treat 3 gallons of tap water use a 1/3 of a teaspoon of our Premium water conditioner. As far as adding too much even if you add 3 times more of our water conditioner than is recommended your livestock will be safe