Hi Scott. Welcome to justanswer.
If you root your phone then you will void the warranty. It will get a 6.0 update when Samsung rolls out the new android OS for this version. You should wait for the official update.
Yes ODIN will root your device and will install an unofficial version of OS. And if you do so you will void the 1 year warranty that came with your Samsung phone as this will update the firmware of your device.
Samsung is soon about to launch the Marshmallow update for Samsung J3, so I would advise you to wait for it.
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