Please contact Support directly with your requirements.
However please note that the may be a delay in their reply due to Chinese New Year (10th - 22nd February 2018(GMT+8) ).
You can check out the PCB order page too.
https://jlcpcb.com/quote/pcbOrderFaq/PCB%20Thickness
You should be OK to continue with your design on the assumption that a <=3mm PCB thickness is achievable as long as you are not also asking for things like 10 layers all with >2oz copper or inner metal layers.
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