will my 3 sim card (australian) work on a iphone?

Test it our first , that's the only way to be fully sure! Although theoretically it should work , if the iPhone is unlocked/sim free.

Ashley Pemberton said:

i am going to buy a iphone 3gs off of someone in south australia. i already have a 3 sim card on plan and i need to know if it will work on the iphone. i have read all this stuff about GSM and stuff but dont quite understand it. anyone that could help it would be brilliant. wouldnt want to buy the iphone if it's not going to work.

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If you want your Iphone 3GS to work in 3 network, you need to check whether the phone is unlocked or locked to any particular network. If it is locked to some network you can unlock it with the help of ClassicUnlocking.com where you need to mention the network and country your phone is locked to.
If it is a unlocked one you can use it with any network provider you wish.