What exactly does it say when it fails at fastboot? It's possible your cable isn't functioning correctly. If you determine that it is a good cable, you could try running the one-click method for installing the sbf, found at [6-21-2012] droid 3 xt862 .906 to .890 1_click_sbf .exe - xda-developers
It says it is for .906 to .890, but I think it will work if you're on .890 to begin with. I then recommend doing the OTA to .906 and re-rooting using the motofail method.
I am using RSDlite, it gets to 15/18 and then fails. Every single time.