I have a Windows Server Standard with 3 Ethernet adapters. I am running a C# application that makes TCP connections to another server. Right now they are all made via the adapter that has a default gateway. Is there a way to make the connections go through all 3 adapters (those are all connected to the Internet) in a round-robin fashion? Connections themselves are quite long TCP sessions (ranging from multiple hours to multiple days).
If you have three different Upstream connections to the Internet, than the proper way would be to terminate the Three Connections on a Business Class router, which support multiple WAN interfaces, and can load balance accordingly. The server will have a single connection to the Router, and one Default Gateway.
If you want to experiment with the Windows server, you can change the Default Gateway, using Netsh Metrics. You will need to set a Loop which keeps changing the Metric for all three adapters during a certain time frame. Here is a sample VP script which is using Utilization to switch the adapters.