The following forum link answers that question.

https://www.dell.com/community/Alienware-General-Read-Only/M17x-R3-CPU-Upgrade-Options/td-p/5573661

Welcome to the Dell Community  @Ross204 

You have a few options.

Intel Core i7-2630QM CPU Upgrade:

http://www.cpu-upgrade.com/CPUs/Intel/Core_i7_Mobile/i7-2630QM.html

The i7-2760QM or the i7-2820QM look promising.

Make sure to update your BIOS before doing any Memory or CPU upgrade.

ALIENWARE M17XR3 A12 BIOS:

https://www.dell.com/support/home/us/en/19/drivers/driversdetails?driverid=9mj02&oscode=w764&productcode=alienware-m17x-r3

Best regards,

U2

EDIT:

Alienware M17X R3 Service Manual (English Only)

https://downloads.dell.com/manuals/all-products/esuprt_laptop/esuprt_alienware_laptops/alienware-m17x-r3_service%20manual_en-us.pdf

Removing/Replacing the Processor Module starts on page 45