Are multiple APA citations supposed to make the same claim or do they combine?
I'm not sure how fully these issues are part of protocol versus just style, but my approach is to try to be clear which works are cited for what purpose (Marks, 2011), which can include separate citations within a sentence (Edgar &amp; Poppins, 1999).

For the example you have there, I would assume you are citing different studies that had different results, rather than three sources that say "there are mixed results": however, I would also expect the later references to refer back to the older results if they are in conflict. I don't think it's necessary for you you clarify any more than that, but if the results of those works are very important to what you are writing, you would probably go into more detail about those particular studies and cite them individually.

To add to my answer based on your additional question in a comment: the citations should be inside/before the punctuation in almost all cases. And I would typically avoid referring to the same work in more than one place in a sentence unless it really made sense to do so (i.e., chronology/natural order). 


  Children like to play games (Ralph, 1967), as do teenagers (Harris &amp;
  Brooks, 1985) and young adults (Ralph, 1967).


I wouldn't consider the following wrong, though, either: 


  Children, teenagers, and young adults all like to play games (Ralph,
  1967; Harris &amp; Brooks, 1985).