Assuming that I'm fluent in an L1 and an L2, are there any studies showing that already overcoming the barrier of learning the L2 aids my process in learning an L3?
Yes, there are studies, and yes, already knowing at least two languages does make it easier to learn another.
According to a study from the University of Haifa, being bilingual does make it easier to pick up yet another language:
"Gaining command of a number of languages improves proficiency in native languages," Prof. Abu-Rabia explained. "This is because languages reinforce one another, and provide tools to strengthen phonologic, morphologic and syntactic skills.
This should come as no surprise, as learning another language has been shown to strengthen cognitive function and make one more intelligent in general (see Does learning multiple languages make you more intelligent?).
There are a number of other internet resources about this topic, see:
However, this article points out that language interference may become a factor, actually making it slightly more difficult to properly learn a language, the more languages you already know.
Personally, I'd say that once you've learned one L2, it's easier to learn another simply because you've done it before. Not learned that language, of course, but learned a second language. Yes, indeed, this new L3 may be completely different from any of the other languages you've ever learned before, but, quite simply, you've learned an L2 before, and that could make all the difference, because you've discovered what methods work for you, how you best pick up a second language, and your mind has adjusted to the idea of placing meaning into a completely different format.