is the only reason it's catching these in their computer due to the 2nd and 3rd flights being the same dates & destinations? in which case if I changed the dates on those, the computer would miss it? because I can have 1 or 2 days flexibility around those flights if it means I don't have to cancel my original booking.

the flights on the first day would have to be on the same day though, although at different times and to different destinations, so I guess it wouldn't take much for a human or a computer to see that this is impossible.

damn technology.