The current part of a Russian language acquisition longitudinal study based on auditory, phonetic and instrumental analysis is devoted to the third year of child's life. We examined the development of supplementary acoustic and phonetic features of the child's speech providing for the possibility for the speech to be recognized. The instrumental analysis and statistical processing of vowel formant dynamics as well as stress, palatalization and VOT development, has been performed for the first time in Russian children. We showed that the high probability of children words recognition by auditors was due to establishment of a system of acoustically stable features which, in combination with each other, provide for the informative sufficiency of a message.