Odin will NOT work with an HTC.
Won't even connect as far as I know, and if it did it'd brick it for sure. Odin=Samsung.
You do need to root and you need to have adb installed as well as fastboot and HTC drivers.
You will also need to have it unlocked through the HTC Dev utility. You can Google that. Its not difficult. You can achieve s-off another way, but I'd simply do it through the Dev site since you lack experience.
As for "stock" gingerbread, what do you mean? AOSP? Good luck finding it. I believe that would be CM7, as 9 is ICS and there was no 8.
If you're referring to the stock ROM it shipped with, you may be able to find that ruu, but you will have to find the one (in all cases) specifically for your 3D. The Sprint won't work.
I'd help you more but HTC is a pain in the butt to work with.
In theory (and from my experience) you simply need to unlock through HTC Dev, download the ROM of choice, root the device, then install custom recovery, put ROM on SD card (either the external or adb push to internal) and the install through recovery.
KTW, its no secret I lack patience with you, so other than this I'm not willing to go further. My best advice is search for YOUR MAKE of the 3D root instructions. If you take your time AND FOLLOW directions, you can do it.
I can't stress that you need to read.
You need adb (probably will be in instructions)
You need a ROM. I'd suggest a vanilla ICS for that device over GB, but its yours so do what you will.