The present paper is concerned with the language development of two young children from two different families growing up exposed to three languages. The children live in Switzerland and have been exposed to English, French and Swiss German from infancy. The focus is on the children's production of these languages, and the contextual and affective factors which have influenced their levels of active trilingualism. The method consists of two longitudinal case studies. Monthly recordings were made by each caregiver (mother, father and third caregiver) in dyadic interactions with the children from ages 2;1 to 3;1. It was found that one child had a high level of active trilingualism, speaking the language of the caregiver with that caregiver over 90% of the time (measured in utterances). By contrast, the other child had a low level of active trilingualism. An analysis of the children's language exposure, such as the position of the community language in the home, the variety of exposure, the interactional style of the caregivers, and the prestige of the languages involved indicates the importance of motivation, largely influenced by the caregivers in interaction, in explaining the children's different levels of active trilingualism.