This study examines the extent to which extra-linguistic factors such as language dominance, order of acquisition and language of instruction are deterministic for multilingual transfer selection and subsequent development. We test two groups of Catalan-Spanish bilinguals acquiring English as an L3 in a controlled setting. We first examine judgements and interpretations of negative quantifiers and negative polarity items after 16 hours of exposure, establishing the participants own baselines for development. Eleven months later, we test those that continued learning English to explore what factors modulate their development. Data suggest that holistic structural similarity is the most deterministic factor for initial transfer selection. Results of TIME 2 reveal that developmental sequencing, however, is dynamic and non-uniform, differing at the individual level by language-dominance in Catalan or Spanish.