How do the girls (Helena and Hermia) react to this mess? In Act 3 Scene 2
In this scene, both Helena and Hermia react to the mess by getting angry.
Helena gets angry because she feels that the other three (Lysander, Demetrius, and Hermia) are making fun of her. Lysander and Demetrius are both claiming to love her when neither of them did before, and Hermia does not believe that Lysander really loves Helena. Because of this, Helena believes she is being made fun of and gets angry.
Hermia is angry because she (first) thinks Demetrius has killed Lysander and (second) because she thinks Lysander is lying about loving Helena.
If you are referring to the effects of the love potion, then they have two distinct reactions. First of all, Helena believes all three of the other characters involved, Demetrius, Hermia, & Lysander, to be making fun of her. Basically, she already knows Demetrius to be deceitful (in his treatment of her early in the play), and obviously can't trust his word despite her deep love for him. It doesn't help, mind you, that Lysander has now pledged his love for her as well. At that point, the only explanation that she can come up with is that they are both using her as sport. Hermia, on the other hand, who has been exhaustively searching for Lysander (who disappeared while following Helena), sees him fawning over Helena and is understandably confused. However, confusion turns to anger when she interprets Lysander's changed behavior as Helena swooping down and stealing his heart while she slept. Helena further misinterprets Hermia's anger as a very keen acting job, and assumes that Hermia is in on Lysander & Demetrius's fun. At that point, a cat fight ensues. :) I recommend the 2000-something film version if the exchange is still confusing. They do a very good job of showing how Helena and Hermia sort of spin into a fist fight by the end of the scene.
I always thought this part of the play was particularly interesting, because at the end of it, the girls are actually abandoned to their fighting as the gentlemen run off trying to beat each other up into the woods. This was a good opportunity for the girls to take a step back and examine what had just gone on (since it is the boys who are under the influence of the potion, not the girls). However, trust between the two girl friends is entirely gone at that point.