Helena: she is Hermia's best friend who is in love with Demetrios
Hermia: best friends with Helena who is in love with Lysander
Titania: the fairy queen, she is very indapendant
Depending on what type of roles you usally play, or enjoy playing, such a character parts, classical leading laddies, or others, is one great way of helping you decide which part best suites you.
Hermia, I would say is the typical leading lady in love. She is kind hearted and not meant to be read into.
Helena, Is also a basic leading lady. She is in love with someone who does not love her back, which is hard for her so she has more dramatic monolouges. She often feels betrayed or tricked and she shoud be well read into.
Titinia, Although she is a fairy she is not always so cheery, she is a very strong induvidual. I class her more as a mild character part although she is also a lead. She is strong not only because she is a queen but she has a powerful personality.
Good Luck with your audition, and also yes these three parts are amazing but there are also alot of great supporting female roles in the show. Its an incredable show and I hope you enjoy it.