Yes, you are on the last red but snookered after a foul so you nominate the yellow as the free ball (red) and pot it, then you nominate the yellow as the colour and pot it then you pot the yellow as the yellow, tada! (5pts)
sorry ellipsis is absolutely correct, this is an old one and I remembered it incorrectly. ok you are snookered on the red, you pot the yellow as a free ball but also pot the red as fluke, scoring 2, then you pot the yellow as a colour and then the yellow as the yellow, total of 6.
Actually I think I like R1Geezer's answer better, but I would qualify it a bit further ...
The last red is hanging over a pocket covered by the yellow. You are snookered on the red and nominate the yellow as a free ball. You hit the yellow and pot the red which is followed in by the yellow (important that the red is potted first!) You then pot the yellow as a colour, then the yellow as the yellow. Three yellows in a row...
My answer was the same as yours, just clarifying that the red had to go in first or it wasn't three yellows in a row ...
First shot: yellow hit (as a red), red goes in, followed by yellow (as a red). Yellow re-spotted. 2 points.
Second shot: yellow goes in again. Yellow re-spotted. 2 points.
Third shot: yellow goes in again. Yellow not re-spotted. 2 points.
6 points total, yellow potted three times in a row ...