Here is part 3 being glued in. I applied glue to both tabs and put in place, starting with the bottom tab. Before applying the glue to the side tab, bend it forward to make it sure it won't come into contact on the wall in the wrong place. Once the bottom tab is in the right place, open the side tab and glue it in the proper place. I used my cross-opening tweezers to hold it closed for a few minutes. I had to add some more glue to the top of the tab and re-applied the tweezers.
The first wall looked good so I went ahead and glued the second wall of part 3. With limited space, I applied glue to the floor plan and slid the tab on part 3 into place. I then burnished it down with the end of my handy paint brush.
I am leaving the last wall for later as adjusting it would put undue pressure on the rest of the part and could move it out of place.