Addressing language learning motivation, Dörnyei's second language (L2) Motivational Self-system (L2MSS) theory consists of three components: (1) "Ideal L2" self which refers to all the language-related characteristics a person wishes to have in the future, (2) "Ought-to L2 self" which concerns the language-related attributes that one believes one ought to possess, and (3) "L2 learning experience" which concerns the immediate language learning environment and experience. Building on this theory, this mixed methods study investigated third language Motivational Self-system (L3MSS) of 40 Iranian EFL teachers, as successful EFL learners (25 EFL teachers with university education in TEFL and 15 EFL teachers with language learning experience in language institutes). They completed an adaptation of L2MSS questionnaire and an open-ended self-report questionnaire. Moreover, six participants were randomly selected to be interviewed. The findings indicated a positive relationship between Ideal L3 self and L3 learning experience of EFL teachers. Furthermore, no significant difference was observed between TEFL graduates and language institute graduates in their L3MSS. Self-report analyses showed that "learning environment" (as a dimension of L3 learning experience) and "positive attitude" (as a dimension of Ideal L3 self) were the most effective motivational factors in EFL teachers' success in L3 learning, whereas "aptitude" and "the impact of people" were rated as the least effective ones. Finally, the interview results were consistent with the findings of the self-report. The findings have certain implications in better appreciation of motivational profiles of successful EFL learners.