In the first part of our paper, we will present the main results of a study about background languages' role (Spanish L1 and English L2) in French L3 interlanguage construction through the analysis of some syntactic transfers in structures like "Verb + (Preposition) + Noun phrase or Infinitive" and "Verb + Object pronoun". We analyzed 484 written productions made in 2013 and 2014 by 28 first-year students of the Translation/Interpretation program of Universidad de Concepción, in Chile. In the second part, we will use these results to outline some didactic principles concerning these particular grammatical structures. We want to propose some work and reflection guidelines to foreign language teachers who deal with similar multilingual teaching/learning situations and we hope we can show them why it is important to be aware of background languages' influence in a third language acquisition process.