The /3/ vowel is spelled in many ways including <eu>, <er>, <ur> and <or>. The schwa is always unstressed and can also be spelled in many different ways. Notice the homograph of divers, contrasting the archaic adjective with the plural noun. I am not sure that the distinction would be made by the majority of native speakers, but this dictionary records it.
This is a contrast between a long vowel and the schwa. They are almost identical in tongue positions, so are normally distinguished by prominence or stress. Very few pairs were found in the dictionary search, and most of these would cause no trouble, except perhaps for the very first one.
The density figure is too small to be worth calculating.
chauffeuse chauffeurs curs cos divers divers earn 'un foreword forward forewords forwards ugh a upturn Upton
John Higgins, Shaftesbury, December 2009