Instead of using 3 tab, go to a heritage style shingle. You can cut them to any length and it doesn't matter cause you won't be able to tell any particular pattern cause of the way they are made.
As an alternative, just gap each shingle a hair/quarter inch more apart to make up the 2 inches at the end. The gap is covered anyway so it won't leak in the top half and the bottom half of the shingle has a natural gap from the tab. Lay them out to see what you think. The gap will help avoid buckling when shingles are packed in to tight.
On the sides, I just run to the edge of the tin anyway. In wind areas, seal each tab an inch or 2 in with an adhesive caulk.