These three vowel sounds are all diphthongs. This means they are actually two vowel sounds that slur together. Notice your mouth move when you say these sounds. Now compare that to "he". When you said "he", you can say the vowel sounds for a long time without moving your mouth!
The sounds of "Oy!" "how" and "height"
Above is an example of someone saying "hi." Can you see how her jaw moves as she moves from saying the first vowel to the second one?
Another example of a diphthong. The word is "coy."