The writer wasnt able to write in enough things into the script for that character to do Chris Klein was in লন্ডন 4 the filming. Klein opted not to return for the last sequel,American Pie: The Wedding, because the script didn't really utilise his character. "Oz and Heather's (Mena Suvari) storyline had ran it's course and he was actually busy doing a play called 'This Is Our Youth' in London
Basically Chris Klein (Oz) was appearing in a play in লন্ডন and the directors thought that there weren't enough he could add too the third movie, hope this solved the question?