There is no modified 3.15 (that I'm aware of). You can only jb with a dongle on that FW.
You might be stuck in service mode if you have a hardware issue with your ps3.
Is your BD drive OK?
If you're confident you have no hardware issue you might be OK downgrading.
Downgrading is risky.
Try everything else first.
You've tried 3 usb sticks. Try more.
Might be a RAM issue. Try unplugging your console and leaving it for an hour or so first before trying upgrade.
Did you have the usb stick in the correct usb socket? On a slim it's the one closest to the drive - not sure on a fat.
Did you have no other media on the usb stick when you tried to upgrade? Should only have OFW on it.
Hope I've helped.
"We all agree your theory is crazy, but is it crazy enough?" - Neils Bohr