Install B2CAppSetup.exe, LG PC Suite IV and related driver.
1. Run LGMobile update
2. Plug your optimus 3d (debug mode on) and wait for the connection
3a. If there is a new version, click on Start upgrading software for lg phone
3b. If you allready have latest version, click on User/Support then emergency mode, insert your IMEI and click on check
4. Wait for download and upgrading (15/20 min)
PS. you will loose root, but you will not loose data, data.img is untouched
PS2. if you got some problems, try again. I've tried few time before have a successfully flash.