Even with two goldfish I would still get an air pump. I have two goldfish in a 55 gal tank and need to use the air pump in the summer as the water doesn't hold much oxygen. I have three fusion air pumps running in my bedroom and can hardly hear them at all. I highly recommend them.
I agree with everyone. Three goldfish in a 40 gal is pushing it. If you want to do that, you should either run two HOB's or get a large canister. Space will be an issue but so will surface area for the BB. I recently helped someone who had a cycle stall because there wasn't enough surface area to support the BB; she had a 40 gal and ended up getting a canister filter. Two goldfish in there should be perfectly fine. As far as the physical space issue, goldfish growth isn't something that we can predict. I've heard breeders say they've gotten both runts and giants from the same spawn. With good food and proper water changes, they will grow to be pretty large and rather fast. As far as which ones stay the smallest, I've heard that ryukin and pearlscales stay the smallest while orandas tend to be larger. But this is not always the case.
And lastly, thank you for doing your research before buying. It warms my hear to see goldfish going to a proper home.