Introduction: This qualitative study investigated morpho-syntactic development in typically developing Tamil-speaking trilingual preschool children (Tamil, English, and Sinhala) aged 4–5 years in the Colombo district. Objectives: To identify key morpho-syntactic structures in each language, examine development relative to typical milestones, analyse cross-linguistic similarities and differences, and describe factors influencing language development. Methodology: Language samples were collected from ten children using general conversation and picture description tasks in all three languages. Morpho-syntactic development and cross-linguistic pattern was analysed using Mean Length of Utterance in words (MLU-w) and detailed linguistic analysis, with reference to available language-specific norms. An intervieweradministered questionnaire was used to obtain demographic information and parental perspectives on factors affecting language development. Audio-recorded parental interviews were transcribed, translated, and analysed thematically . Results: The children achieved higher MLU-w values in the languages with greater exposure, while production of complex morpho-syntactic structures was limited. Frequent codeswitching, code-mixing, and morphological nativization of borrowed words were observed, reflecting systematic strategies in multilingual development. Factors influencing second and third language development included degree of language exposure, individual child characteristics, and home and preschool environments. Conclusion: The findings indicate that while MLU development was generally appropriate in children’s dominant languages, in-depth morpho-syntactic analysis was necessary to determine mastery of age-appropriate structures. This highlights the limitations of relying solely on MLU measures, which may mask weaknesses in complex morpho-syntactic development, and underscores the need for comprehensive, language-sensitive assessment practices to ensure valid evaluation of multilingual children.