Looks like Steve's away for a bit.
Have you tried the WDM or ASIO driver mode with the Rock Frog?
Unlike most units of its type, it appears to support both. If you can get away from MME, you can reduce or eliminate that echo.
ASIO will alow only one active sound card, so if you use the ASIO mode, all of your input and output would have to go through the Rock Frog. That's where signal path comes in. Guitar>RockFrog>USB IN>Software(GTP3)>USB OUT>Speakers(Headphones).
WDM is slightly more flexible, and would allow you to send the output to your computer sound card and speakers.
Depending on what that sound card is, that may or may not be a good thing.
Since the Rock Frog claims to support ASIO, I would try it. If you can get your latency under 10ms, this is usually workable.
10ms is just at the threshold of perception, so you would probably notice it, but you can get used to it, and it's not unusable.