This thesis is a case study of simultaneous trilingual acquisition examining a child's (S) development of English as she acquires English, Italian and Scottish Gaelic simultaneously from birth. It investigates the effect that the acquisition of three languages from birth has on the development of the child's English. S's trilingual language development is examined to see how it correlates to monolingual and bilingual acquisition. Specifically, this thesis reviews S's MLU development and her general development, whilst focusing on phenomena that are salient in the acquisition literature, and compares it to the language development of monolingual and bilingual children presented in the literature. It is clear that although S's development of English is largely in keeping with that reported in monolingual and bilingual acquisition literature, it differs to her monolingual and bilingual counterparts in relation to target deviance. The target deviance observed in S's data is examined in order to provide evidence for cross-linguistic influence in multilingual language acquisition. The first phenomenon analysed in detail looks at S's data only whereas in the second phenomenon, elicitation tasks involving S and her bilingual peers are carried out to support the target deviance observed in the spontaneous data. Grammaticality judgement tasks testing S's English and Italian are also used to examine target deviance in S's grammar. There are four prominent theories of cross-linguistic influence in multilingual acquisition and this thesis examines the various target deviant phenomena that can help us to understand each aspect of this phenomenon that has received different explanations in the literature. Overall, the results of this case study show that cross-linguistic influence is a characteristic of multilingual language development. I propose that cross-linguistic influence occurs as a result of the three languages in S's triad being activated simultaneously, i.e., when a structure that is grammatical in S's other two languages surfaces in her English at a stage when S's inhibitory control skills are not fully developed.