The /3/ vowel is spelled with <er>, <ir>, <ear>, <ur>, and <yr> in myrrh. The /ʊə/ diphthong is spelled <our> and <oor>.
This is a contrast between a long vowel and a diphthong. The starting positions are fairly close, but there is a difference of lip-rounding, so it causes no problems for learners.
Interesting pairs include:
The density figure is 1.33%. The set makes 10 semantic contrasts giving a loading of 71.4%.
burn bourn burns bourns burr boor burrs boors gird gourd girds gourds myrrh Moor pearly poorly per poor Shirley surely spur spoor spurs spoors turd toured year you're
John Higgins, Shaftesbury, December 2009