Trilingualism is generally treated in the relevant literature as another type of bilingualism, and theories and findings from studies of bilinguals are often assumed to be applicable to trilinguals by extension. Trilingualism is frequently explained briefly as a special phenomenon of bilingualism, using special cases of brain-damaged trilinguals who recover all three languages, or of young children who are precociously trilingual. There are many types of trilinguals: children growing up in a trilingual environment, adults living in a trilingual or multilingual community, and fluent bilinguals who have learned a third language at school or for other reasons. Most of these types do not have much choice of whether they wish to be trilingual; it is simply a fact of their particular circumstances. How they deal with three languages is interesting in that the three languages (or cultures) cannot be 'balanced' or equal, as they can be in a bilingual person.
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Suzanne Barron-Hauwaert (2023) studied this question.
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