If there are three vowels in one syllable we speak of a triphthong. In most cases the triphtong is formed with semivowel + full vowel + semivowel. It is well possible that there are languages with three full vowels in one syllable because the main proberty of a vowel is that an infinite number of vowel can follow each other. (What is not the case with consonants. We can't pronounce mtrkts, but we can pronounce aeoueau.)