I'm still tinkering with positions but I think I've chosen my final three stones now. I've been experimenting with 5 and 7 (1 big, rest small) stone compositions but have decided to stick with three for the time being at least. I like the simplicity and am trying to visualize the look when fully planted. Vision, or lack of, is a weakness of mine.
Excuse the colour difference, they should look the same when they're all wet.