Over the last several years, we have been witness to a growing body of work that examines the acquisition of Portuguese as a third language (L3). Here in the United States, Spanish speakers account for 45% of students enrolled in Portuguese classes (Milleret 2012), divided among first language (L1) Spanish speakers, second language (L2) Spanish speakers, and heritage speakers. While these three groups are all speakers of English and Spanish, they differ with respect to the order and context of acquisition of the two languages. In this essay, I propose that access to these three linguistic profiles in Portuguese classes offers a unique opportunity for us to study third language acquisition here in the United States that arguably has not been afforded elsewhere. In L3 acquisition research, a primary interest is in the differences in acquisition processes when comparing learners with a mirror image language pairing (in this case, L1 English/L2 Spanish compared with L1 Spanish/L2 English). More recently, we have also begun to examine how mirror-image groups of sequential bilinguals compare with early bilinguals (in this case, heritage speakers of Spanish). Herein, I review research questions that drive the field and illustrate how we have addressed these questions via examination of L3 Portuguese acquisition.