Either way, into the fray! The adhesive mucus being launched in all directions might have given One trouble before, but now he's moving nearly twice as fast as normal thanks to the sickle's magic, so I think it's fair to say he manages to evade the frenzied attacks easily enough.
On the downside though, the short range of the weapon pretty much ensures the Fyarl will be able to flail a claw at him once or twice before One can get in an attack.
In that case, no, the blade probably doesn't inflict too much damage. On the plus side though it does help unbalance the beast, which in turn makes dodging the backhand a piece of cake.The Fyarl is distracted and again does not counterattack against the oncoming triplet, but the blade probably won't do much damage. Only a specific type of material can cause lethal damage, so chances are the blade will not dig into the armor deep enough to cause serious damage. It will grunt in pain and try to deliver a powerful backhand slap against First with enough strength send a normal human flying several feet.
With a sharp step backwards First avoids being crushed beneath the falling demon, as well as putting himself directly behind it where the mucus-blasts shouldn't be able to reach him*. Turning the point of his blade towards the Fyarl's throat he stabs downward with all his genetically enhanced might, hoping the creature's armor might be weaker at the neck.
Alpha's hit full in the chest and thrown back into a nearby dune, sand cushioning the landing somewhat. Letting out a strained grunt as he tries to breath and dull pain blossoms in his lungs, Alpha begins struggling back to his feet, movements beleaguered by the two broken ribs he's just acquired.The Fyarl's attempted attack against First will probably fail due to the synchronous attacks of both First and Alpha. In this case, the pain felt in his leg will cause the demon to lose focus and fall backwards. Surprised by this turn of events, it will attempt to kick Alpha away (which will almost assuredly hurt a lot provided it succeeds, as the Fyarl's kick is more powerful than the combined strength of three horses), and begins to just shoot streams of mucus at a wide array of angles.
*Unless the Fyarl can pull that Exorcist thing where it rotates it's head a full three hundred and sixty degrees. Then he's screwed.