I believe they will get their cutie marks some point in the show. I think they'll still all be friends after they get their CMs- I'm pretty sure those three of all ponies won't ever be out of ideas or bored.
Someone people are saying that they all want to get the same CM but I don't think thats true, I think they're just testing out activities and hoping that it'll make one of the ponies discover their hidden talents.
I wonder, however, what they'll do when/ if one of the three gets their CM before the other two, I could see that creating tension in their friendship.... Anyways, I love the episode featuring the CMC, I think their storylines are all adorable.