Tricky one, but doable if you work systematically. You might want to look at my guide to
beamer overlays (also available as a TUGboat article, currently only to TUG members).
What you need to do is use the idea of relative slide positions, indicated by
. and numbers in parenthesis. I'll explain the detail in comments in the code (trying to gie this up-front is tricky):
% First item on slide 1 onwards. The `+` will be replaced here by a `1` here:
% of course if you add material before hand absolute value will change!
% Second item on same slide as first: `.` repeats the previous number
% Third item appears on third slide: `+` is slide 2 here, so `+(1)` is slide 3.
% Repeat the preceding `+` (`2`) but back-up one to `1`, with the second `.`
% replaced simply by `2`.
% Again, repeat `2` (the last value `+` stood for)
% Auto-increment `+` to `3` and insert
% Repeat `3` again
% Two simple auto-increments
Done this way, nothing is hard-coded so it should be possible to use the code flexibly.
(To do this sort of thing, I'd start with the hard-coded numbers then replace one at a time by the relative specs.)