How to search with multiple parameters

Hi guys, I need to put a search form with three parameters, and I already did. My problem is: when I search defined by those three parameters, search is done correctly, but when I choose one option only, I didn’t get success, in other words, any results are returned. My search form is composed by 3 selects: 1st level taxonomy, child taxonomies (returned according to first select through ajax query) and the last is the post title, and I need to allow users search only by one or two parameters, without necessity to fill the third. How can I do this? Detail: that search must be done in a custom type post, so, I put a hidden field passing my custom type post value.

Note: I tried putting an “if” to verify the “s” url variable, but didn’t work.

Thanks.