i just cleared vault 3, killed motor-head, and freed the random slaves... but cannot find Col. Hsu's missing Ranger tht he asked i help find... how do u find him?

If you killed Motor-runner before talking to the ranger, he wont be there. You can still finish the quest though, just go back and talk to the Colonel.

where is he to talk to him?

In the living area of the vault there will be an area filled with traps, follow that corridor and you'll find the room the ranger was/is in.

Up a stairwell that is trapped (he trapped it) close to where the caravan hostages are. I think you have to lockpick the door but I might be wrong. You will see a dead fiend right by that stairwell who apparently got blown up by one of his traps.

I seem to remember a dead fiend laying in the hallway where the traps start too.