I used the HM-3 in a band I was playing in many years ago as the main distortion sound (the band had only one guitar); it worked surprisingly well and was quite defining for that band's sound; most importantly it set us apart from all the other local bands.
For me the hm3 is definitely the best of the boss metal pedals, I can see how it worked for you.
The metal core is also ok.
I forgot I also have the mz-2 'digital metalizer' which I don't like at all on guitar, it's maybe the worst boss metal pedal. However, it is pretty good with a techno synth. Very filthy.
This thread is making me think I should get a metal zone and complete my set ;)
I hope Agent Orange that you enjoy your HM2 :)