Working as Intended?

So I recently qued up as heimer top. And after making a set of masteries, I took thunderlords to test and see if it would proc off of his turrets? However I was surprised to find that it can proc off a single E. Since each rocket is considered a seperate spell, as long as you land three it will proc. Is this intended? have to say it was satisfying landing that e and watching a purple lightning bolt rain down on my enemies.