Windows (10) on 3rd Partition of MacBook Pro
Here are the two issues you are facing:

Issue #1:

Disk utility shows you only two partitions, but in reality you have four; an EFI partition, and the OS X recovery partition are both hidden. Since Windows XP and Vista didn't support EFI in those days, Apple had to emulate a legacy BIOS to support Windows on that model. A legacy PC BIOS only supports MBR partitions, and MBR disks only have four entries in the partition table. Bootcamp can't create another partition that Windows would be able to see.

Issue #2:

Microsoft didn't officially support installing Windows from USB until Windows 8 came out in 2012. It was totally possible (and even easy) before that, but since Microsoft didn't support it, Apple didn't support it either. After Apple ditched the optical drives in their computers, USB was the only other way to get Windows on there, so finally Apple started supporting it. But they didn't backport the fix to earlier models so you're stuck with installing from DVD.

So here are your options:


You can sacrifice that 2nd partition you have and then Boot Camp will work normally.
You can delete the recovery partition. There are guides online on how to do this. It involves the command line. Also you have to flatten your CoreStorage volumes. Apple definitely doesn't support this but OS X works fine without it as long as you do it correctly.
You can find an older version of the Win10 ISO. The version you likely have is the Anniversary Update (build 1607). If you can find a 1507 or 1511 build, those will fit on a standard DVD. Once installed, you'll be upgraded to 1607 via Windows Update.


One more thing...

There are also guides out there on installing Windows in UEFI mode on a Mac. Don't do this. Your model doesn't support that and it's more trouble that it's worth. You'll run into video and audio issues (if you have dual GPUs).

Hope this helps!