As you might have guessed, this approach likely is studying a topic with the most existing research, when compared to the other two.[4]
Though I think I do understand the author's intended point here, I think the wording is perhaps a bit tricky and potentially confusing for some readers. To make links explicit and content easier to understand, consider revising to something like:
"As you might have guessed, this approach is more likely to be used if you're studying a topic which already has some existing research in the literature base."
(I think this section will be clearer, and can stand on its own, without the last "...when compared to the other two" part of the sentence.)