Erorr message saying Not enough RAM to run minecraft server, but I have enough?

"Error occurred during initialization of VM"
"Could not reserve enough space for 3097152KB object heap"

I have 8GB RAM, 7 Usable
I'm only using 3.1GB of ram so I should be able to run the server with 3GB of RAM but it won't let me…

Thanks for your help.

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