Question

a pettiness... and slowly,very slowly,as if thrice adream... please explain

Answer 1

The poet describes the slow movement of the snake. The snake was crawling back into its hole, very slowly. The poet feels that the snake is in a state of dream. It has no anxiety or hurry. 

Answer 2

This show that snake is calm composed and unmindful of its surrounding.He is actually in a dreamy mood,in its own world.It is like seeing a dream in a dream.

thrice adream can also mean completely satisfy