Well, there is an entire study on this!
Salaberry concluded that the participants with advanced knowledge of their L2 outperformed the participants with low L2 proficiency in the acquisition of past tense aspectual markers in the L3.
So, keeping your L2 can help with your verbs in the L3. The more of the L2 you know the better.
She also noted that the learners
with a higher L2 proficiency scored higher on all of the presented tasks, yet there were still other
factors to consider such as amount of L2 input received and the activation of L2 during the study itself.
Still, those more proficient in L2 are more likely to do better on all the tasks given, give or take a few due to other factors involved. This quote is from another study in the actual study.
They concluded that advanced knowledge of L2 English, (over advanced knowledge of
L3 Dutch) was the best predictor of the participants’ accuracy of learning the new L3 words
Surprisingly, the amount of knowledge of L2 can better predict your learning accuracy of L3 words instead of your knowledge of L3 itself! This quote was found in the study from another study.
The results only further proved that being more advanced in your L2 can help with your L3 tremendously:
The high L2 proficiency group produced target-like pronunciations 94% of the time, and an L2-
like alveolar trill the remaining 6% of the time. There were no instances of L1 like pronunciations for
A 94 percent accuracy was shown when the group with high L2 proficiency was told to pronunciate 16 different target words from the L3. Thus, for a better L3, keep your L2 and make sure you are very proficient in it.