This study considers the upper limit of ultimate attainment in the L2 French and L3 English of trilingual learners. The learners are native speakers of Moroccan Arabic who started learning L2 French at eight and L3 English at 16. They are advanced in both languages. Four constructions representing the verb movement and null subject parameter were tested using acceptability judgement and preference tasks. The results show that the advanced L3 learners performed comparably to the native speakers on both parameters while the advanced L2 learners were less accurate. We conclude that nativelikeness is possible in L3 ultimate attainment even if learning starts at post-puberty and takes place in a formal foreign language context. However, convergence with the native knowledge may not affect all the component structures of the parameters as the learners' ultimate attainment in L2 French shows.