I haven't seen it for a long time, but I don't think it has split. It should have Trim. So what you'd do is start with the large one, Trim the first 1/3 of the video and save it with a new name. Then reopen the large version and repeat with the 2nd part, and repeat again with the 3rd part.
I'm pretty sure the higher-end editor, ShowBiz, has a Split function simplifying this process.