Congratulations, you have certainly got your hands full there!
Do you mean to tell which two of the three are identical, or how to tell the identical two from each other?
If the former, I presume that all 3 look similar - you should find that as they get older, one will start to look different from the other two, but you as parents will probably find it more difficult to tell as even the two identical ones will look different to you. I would wait a few months and the get the concensus of opinion from your friends and family. Is there any particular reason you need to know?
If the latter, tattoo one of them? - only kidding ;o). Using different colours of clothes and making sure you stick a name on their nappy? again you should find that you notice the differences rather than similarities as they grow older.