This presentation will revisit the current theoretical models attempting to explain the developmental process of third language acquisition and the factors affecting it. There are currently three active models on the market, each with some empirical support. The critical research questions being debated are whether the L1 or the L2 structure has an effect when acquiring the L3 and whether the totality of the impacting grammar is transferred as an L3 initial stage. A further debate is whether transfer from a previously acquired language can be only facilitative, or it can be detrimental as well? I will review evidence for the current models and will propose a new approach called The Scalpel Model of L3 Acquisition. It has many features in common with another recently proposed model, the Linguistic Proximity model. Both these models essentially argue for L1 or L2 transfer, based on grammatical considerations or structural similarity, not necessarily typological similarity. They also argue for facilitative as well as harmful Transfer, and against wholesale transfer of the whole impacting grammar. I will discuss the implications of these theoretical approaches to acquisition in a language classroom. My discussion will be specifically focused on Basque-Spanish bilinguals acquiring English and other third languages.