You've got the concept right. With Shimano triple setups I think of the clickstops (detents) this way: 4 primary positions plus an additional trim position from the extreme left side. When adjusting the cable I've found that usually it's best to have no slack in the cable when in the extreme left position with all of the cable out of the shifter. I think the problem you previously had with the shifter jumping from the 3rd position to the 1st position was from too much cable tension. Some setups seem to work best with a small amount of cable tension when in the lowest (first) position. YMMV. If you are able to use all of the normal gear combinations with no chainrub on the derailleur then you are good to go!
Al