We know that 280 ft^2 is painted with 4/5 gallons of paint, or 4/5 gallon per 280 ft^2. We want to paint a certain number of ft^2 with 3 gallons of paint, or 3 gallons per x ft^2. We can write a proportion: 4/5 = 3/x. Cross multiplying, we get 4x = 15. Dividing both sides by 4, we get x = 15/4 = <<15/4=3.75>>3.75. Therefore, Stacy and Neil can paint 3.75 ft^2 with 3 gallons of paint. Answer: \boxed{3.75}.
