What size pond filter system for 3000 g pond?

you'll need a large pump capable of a couple thousand gallons an hour, and a filtration system that screens out solids, breaks down the nitrates/nitrites biologically, and has a

a uv light to kill of free-floating algae & bacteria. . .

you'll also need aerators in this large a pond to keep from having dead zones, and if you live in a region of winter freezes the aerators will help keep the top from freezing

alot of work and expense but well worth it! my koi pond is only about 300 gallon but we enjoy it year-round

there are many online sites that can help you answer your questions

good luck!