I have 3 Filter to search in title
my.\nice.\title.\one i found 30 entries
another.\nice.title.\two i found 20 entries
and.\a.\title.\three i found 40 entries
The \ are needet that it works. No problem.
But when I insert this three filters in one, I found not 90 entries. I have checked that not all filters must pass.
are you testing with filter dialog testing tool? it might not be entirely accurate, to see if filters are actually working properly use feed debugger (hotkeys f D or actions menu -> feed debugger)
there you should be able to see which filter rules apply to which articles.