Which Kohen throws the hyssop ... into the fire?
Targum Yonasan: A third Kohen.
What is the significance of the three commodities?
Rashi 1# (citing R. Moshe ha'Darshan): They represent the three thousand who fell by the Eigel.
Rashi #2 (citing R. Moshe ha'Darshan): The cedar is the tallest species of tree and the hyssop, the shortest. It therefore serves as a Si'man that someone whose vanity causes him to sin, should make himself small like a hyssop and like a worm, 1 and he will be forgiven.
'Tola'as' means, both a 'thread' and a 'worm'.
Why does the Kohen add "S'reifas ha'Parah"?