Triple Speed Ethernet MAC core usage

What you are planning to do is feasible, but I really don't know how to do it.

You need to pre-configure all tse registers or have some device which make this for you after fpga design has loaded and this is what I do with Nios.

PHY is a lesser problem since you can use the standard configuration or you can use configuration strap pins which are usually available.

You need better support than what I can give you.

Cris