How big is three acres?

A foot is a unit of length, an acre is a unit of area. You can't convert that.

1 acre is 198 by 220 feet or 43560 square feet, 4840 square yards and 6272640 square inches.

So the answer to your question, how many ft is 3 acres would be 594 by 660 feet or 130680 square feet.