Triphthongs in English

A triphthong is a combination of three vowel sounds. They are pronounced together and form one syllable.

[aɪə]

The longest of three sounds is "а".
In writing it is expressed by letter combinations "ire", "yre", "iar", seldom by "ier"

ire - fire ['faɪə] 
yre - tyre ['taɪə] 
iar - liar ['laɪə] 
ier - tier ['taɪə]
iet - quiet ['kwaɪət] 

[auə]

In writing letter combinations "our", "ower" are used.

our - sour ['sauə] 
ower - power ['pauə] 

[juə]

In writing letter combinations  "eur", "ure" are used.

eur - European [ˌjuərə'piːən] 
ure - pure [pjuə]