I tried entering both of those in the level 1 TOC box but I get a popup saying "Xpath expression is invalid.
Just use the expression builder button to the right and select h3 from the drop down menu--instead of trying to create the expression yourself. For h3 the expression that the builder produces is:
The chapter divs would also work. The builder produces the following expression:
//h:div[re:test(@class, "chapter", "i")]