I opened up creature NVCompanionEdEUpgradeWeapons in GECK and
under template data selected the actor base of RL3 follower, and clicked
off all boxes below that except for "use factions". The name under the ID
window changes to "Sergeant RL-3". In the drop down "script" window under
"name" I can't find a suitable script so I select "none".
After I do this, under the "sounds" tab I now see sounds from "mister handy"
listed. I don't know how to change this as this window is now grayed out.
I end up with something that appears in game as RL3 but does not fight
or use RL3's voice. Can someone tell me what I am missing here?

RL3's voice is not included in the game files, and we as modders are not allowed to transplant them from Fallout 3 into NV.

As far as the fighting part, well, you'd need to be sure he's got the right weapons, and the right AI, etc, and... well, move in that direction.