"they come in a slow march, a band of three" Shaw is comparing the turkeys to a band, he is trying to set the scene, telling the reader/listener that the wild turkeys are together as they come in a "slow march". This is ironic because wild turkeys wouldnt be so organized as to march in a straight line, as a band (with instruments) would.
"three emissaries of shadow" Shaw is saying the three turkeys are messengers of shadow (again, "shadow" = darkness = death).
Examples of simile:
"wings, going up with uncanny lightness, almost as though (as though = simile) inflated"
Shaw uses simile hear to propel his diction. He wants to describe the majesty of the wild turkey, the sense that yet it is such as awkward bird