From English as L1 to Portuguese as L3 through Spanish as L2: transfers in verb regency/transitivity, with special emphasis on prepositions handles topics that have not yet been deeply explored. In addition to a unique combination of languages, this thesis focus on a specific phenomenon and grammar item, thus providing an important contribution for the development of an extremely recent (and promising) research area - Third Language Acquisition. Although TLA studies are incipient in Latin America, in Brazil one may see the urge to adapt a new linguistic reality as more and more Spanishspeaking North Americans who have come to the country want to learn Portuguese. That would call for some investigation on the influences between previous learned languages and the target one. Theory includes contrastive studies, particularly transferences among the three tongues selected; moderate linguistic relativism; the functionalist school; modern transitivity concepts, especially Valence Grammar; Error Analysis (Transference Analysis); bilingualism / multilingualism definitions; and TLA influence factors. An underlying consideration is the fact that the acquisition of a third language is more complex than the process to acquire a second language. By means of some tools (questionnaires, compositions and elucidative activities), research reporters were selected; the most common errors caused by English (L1) / Spanish (L2) transferences into Portuguese (L3) were diagnosed; and the results were then compared against the proposed hypothesis. Data analysis indicated that the better expertise one has in Spanish, the higher is their Portuguese influence as the amount of L2 transference in L3 rises along Subgroups (C - low; B - average; A - high). Meanwhile, English influence in Portuguese performance decreases and interference neutralizing rises. It would not be possible to affirm, however, that Spanish command is essential for L2 to be more frequently employed than L1 as a basis for transference, given that English indices are still high in the best command subgroup. Despite Spanish pros - higher typology proximity to TL; FL status; and the last language effect -, ones MT prevalence may result from verb transitivity choice as an object of study. Another target that has been achieved by this thesis is the formulation of transference matrices that may help Portuguese as Foreign Language teachers in their classrooms. Once they are aware of agreement differences, they will be able to come up with specific activities to help these students use Portuguese prepositions correctly, instead of encouraging them to transfer directly from one language into another - unfortunately, most PFL grammars does so. For the future, it would be interesting, for example, to develop a similar study with other constellations of languages or to choose another object while keeping the profile selected in here. By and large, this work intends to enrich the academia by collaborating to describe Portuguese as a third language (rather than just a foreign language), as pioneer handling of topics and the results that have been achieved should qualify this research for future reference.