To try to discipline three girls who are not yours, and who may have been spoiled by their mother or father is not at easy task. First of all, you need to win their respect and get close to them, by being interested in them, getting to know them. And most important, you will have to work with the mother on this. Otherwise, this could be a very unhappy family situation.
Just be sure not to act like their father if they don't want it. I dont know the story, but a new stepdad should be respectful to the children. Im sure it's not easy for those girls to not have their parents together. My stepdad is often rude.
You should let mom handle the discipline otherwise you will alienate the girls and potentially cause more problems. I would suggest building a strong relationship with them before you ever try to intervene. Let mom know what's going on and let her dole out an appropriate consequence. Perhaps also sit down and have a family meeting letting the girls know exactly how things will work... That even if mom isn't present there will still be consequences and be very clear about both of your expectations for their behavior. Be supportive of your wife and if she has difficulty being consistent, use gentle reminders rather than criticism so that the kids behavior doesn't cause a rift between you two.
Make sure that they like you! My best friend HATES her step-mom who shes have for about two years! She is still like a total stranger to her! She still does chores. They won't do anything for you unless that they know that they love you. They should know that they're only doing this because you love them. If you really can't get them to do anything, start taking away privileges.