After leaving the Army base, Zain had accepted the fact that he might not get the diamond reward. It was why he was so hesitant in the first place, but due to the quest details itself, there was a slight hope that there was no need for him to kill all three.
Originally the diamond quest had stated for him to only find the active meteorite, and after finding it, the real quest had begun. Thankfully, the military along with the Zombie Hunters were able to beat the last demon without their help, and in turn the quest had been completed.
'I wonder what the diamond card will give me. The rewards have been all over the place once they hit gold, so it should be something quite valuable.' Zain thought. 'But, the diamond quest was easier than I thought. I wonder, is it because I was given that quest a long time ago.
'Is that's the case, maybe I've just improved a lot, or it was the fact that everyone else just happened to be there when I discovered the meteorite.'