I would just go replace all three fans. Also how long is the heatsink? I'm assuming the 80mm fan wont cover the entire heatsink, which is why I recommend buying the three individual fans. However if two 80mm fans cover the entire heatsink then I would go for 2 of those fans if you can manage to mount it and secure it without it being loose.
yeah, i went ahead and tried attaching an 80MM fan i had laying around, unfortunately it's too tall and runs into my sound card, and the connector is a bit smaller than normal (which is easy enough to get around)