Have a word with the guys at Spacephone Symi. It's easy to find opposite Meraklis taverna. They are very helpful and will sort out a SIM card for you, although with all the places with free WiFi on the island these days you may find you don't need to pay for 3G at all.
Their phone number, if you want advice before you get there, is 2246 071596 and they have a facebook page so you can message them from there if you do the Facebook thing: facebook.com/spacephone.symi