So you have a dual Etherchannel/port-channel between your switch and 3900?
On a switch, you'll want to select the hashing algorithm that uses the most "changing" attribute(s). Often src-dst-ip offers a good distribution (which I suspect was meant by Mark when he suggested srt-dst-port), but if the bulk of traffic is between the same two hosts, it will send all that traffic to the same link.
On your 3900, unless you're using a switch module that supports various hashing algorithms, you might be limited as I'm unsure hashing algorithms choices are offered with routed ports.
Understand even with the "best" hashing algorithm choice, a single heavy flow will only use one link. Also understand, hashing doesn't take into account current link loading. I.e. two heavy flows might be directed to the same link while the adjacent link is effectively unused.
