I haven't heard the Heir Audio 3 myself, but I've read about it through reviews. It sounds like a warm IEM with a decent bass presence. In contrast, the Merlin is all about its bass heavy sound signature, coupled with some really prominent and detailed treble that bordered on sibilance. With almost equal emphasis on highs and lows, the Merlin was a very good choice for EDM genres such as melodic trance.
In fact, when deciding between the Miracle, Merlin and Mentor, I was torn between the latter 2, but chose to go with the Mentor for its more balanced sound signature. You lose a lot of bass authority and weight with the Mentor, but you get a more present midrange and better controlled treble. The highs on the Merlin were difficult to tame at times, and it sounded a little like the Westone 3 to my ears.