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:
pearly poorly
Shirley surely
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