Depending on what firmware your Surface Pro 3 has loaded, you may not be able to PXE boot at all.
http://blogs.technet.com/b/deploymentguys/archive/2013/05/16/pxe-deployment-with-surface-pro.aspx
If the device your trying to image still has the April firmware, you won't be able to PXE.
You can either use a usb boot drive to chain into MDT, or update the firmware while on the factory load, then PXE.