Early trilingual development is an excellent testing ground for input reduction effects on acquisition outcomes. This article reports a study investigating input-outcome relations in a child Leo in Hong Kong, who was addressed to in Mandarin, Cantonese and later also in English by caretakers through 'one caretaker-one language' and 'one day-one language' practices. Caretaker-child interactions were recorded monthly from 1;6 to 2;11 and compared with monolingual baselines in grammatical complexity, lexical diversity and verb morphology. Receptive vocabulary tests were administered at 3;1. Results show that Leo, having accumulated 54%, 26% and 20% of his total input in Mandarin, Cantonese and English respectively from birth to three, demonstrated monolingual-like development in Mandarin and surprisingly, also in Cantonese in most lexical and grammatical measures. Hearing English primarily from non-native speakers, Leo was able to match monolinguals in receptive vocabulary and develop productive morphosyntax in English. The findings suggest that language distance and similarity should be taken into consideration in predicting developmental rates and outcomes of specific languages in trilingual development, and that the role of language input from non-native speakers in the acquisition of a third language warrants further examination.