Speaking in a foreign language involves learning phonemes that may be influenced by previously learned sounds. Recent research reveals that mutual phonetic influences are not limited to L1-L2 transfer as once assumed, but also exist among the L1, L2 and L3, emphasizing the complexity of interlanguage. The acoustic properties in foreign language speech are understudied in the Philippines, where English is often an L2. In order to determine possible L2 effects on L3 phonology over time, this study examines changes in both vowel quality and quantity in a population of 22 Filipino students learning German. Eleven students had an intermediate level of proficiency (B1), while the rest were beginners. The study reveals that many interlingual vowel pairs shared overlapping phonetic spaces, with some exceptions for back vowels. The role of proficiency in German was more significant in distinguishing vowel durations between L2 and L3 and producing distinct L3 vowel pairs.