Test all 3x sticks alone, and also test all 4 slots. Sounds like 1 will fail. It happens. Sometimes reseating the ram is all that's required, it'll re-scratch the pins on the ram. Sometimes it's a slot that's not working right, so any ram on it fails.
Only by running the full test of what's what with all the sticks/slots will you be able to say for sure if either 1 slot is bad or 1 stick. If it's just 1 stick, replace it with another or another set, if it's the 1 slot, use the other 3 until the mobo fails entirely.