There are three positions for adverbs in a sentence, front, middle and end. Front and end mean what they say. Middle can be either after the first auxiliary verb, after be as a finite verb, or before any other finite verb if there is no auxiliary verb. Linking, viewpoint and stance adverbs generally occur at the front; degree, frequency and adding or limiting adverbs generally occur in the middle; and manner, place and time adverbs generally occur at the end.
The difficulty in your example lies in deciding what kind of adverb potentially is. It doesn’t fit readily into any of the above categories. It could be seen as a stance or viewpoint adverb, in which case it would go at the front, as in (1). However, I’d be inclined to see it as a limiting adverb. It would then go in the middle, as in (3), where it appears after the first auxiliary could.