You have to re-word your intentions into instructions for a computer program that interprets what you say very literally. The logic can be tricky but experiment a bit and you should figure it out.
Here’s a smartlist example that’ll fail adding a 3rd rule
Match if ALL are true:
1. Artist is (Rolling Stones)
2.Year is less than (1975)
3.Artist is (Beatles)
Unless you have a song where both the Rolling Stones and the Beatles are the artist (none I know of), you’ll have an empty list.
Your intention may have been to create a list of classic songs by your two favorite artists. But iTunes can’t find a song done by both artists, so it returns an empty list. Switch to match “if ANY are true” and you’ll get that classic songs list you’re after.
BTW, Not sure what you mean by the 3rd button. My version of iTunes (11.0.2) doesn’t have one.