Which G3 iBook? Colored or white?
The white ones can, so long as it's got a VGA connection (no DVI). It'll only mirror by default, so you'll just be seeing the same stuff on both screens. However, most (but not all) G3 iBooks could be altered to allow monitor spanning (different stuff on each display). Check out the application "Screen Spanning Doctor" for easily enabling this functionality (make sure your iBook is supported).