Heya, hope I can help here!
3, 6 and 5 would create a length of 14 and give “No doors” (3), “2 Doors” (6) and “1 Door” (5). You don’t roll again on the Section table - you just use the already rolled dice. (A section that is 1-3 squares in length would indeed always have no doors!)
In order to get doubles on the section dice, you would have to get something like 3, 5, 5. A “5” in this case indicates a Trap of some kind in the hallway.
Triples would be something like 3, 3, 3. In this case, “3” would indicate a Feature, but since there were triples, it also indicates treasure.
I hope that helps! If I get the time, I’ll make a video showcasing how to use the dungeon generator!