Hi Barbara! S&H is set up to work opposing muscle groups per session. Since legs and shoulders didn't fit into that equation as specifically, they were put together for their own day.
When working opposing muscle groups you can lift heavier for each body part since the muscle fibers recruited for the first body part do not interfere with the muscle fibers recruited for the second body part. Now even though legs and shoulders do not oppose one another, you can still go heavy with this grouping because again, the muscle fibers recruited will not interfere with one another.
You can do the order that you listed above but I would definitely put a day of recovery between them so that all muscles get some down time before they are used again.
I want to point out too that certain shoulder exercises use the tricep muscles (ie: overhead presses) and certain tricep exercises use the shoulder muscles (ie: dips). So you might feel as if your fatiguing a little sooner than usual by doing shoulders and triceps together.
Looking at your line up, I think it would best work in this order....
1) Chest and Back
2)Legs and Biceps (do legs first for a better warm up to the muscles)
3) Shoulders and Triceps
Good Luck!