Your system is in flex mode because your channels are not of the same capacity. Since you have three chips but a dual-channel board/CPU flex is the best you can do. You must have an equal capacity on both channels to run full dual channel.
in your case, it it probably best that way. the additional ram is likely worth the small degradation in speed from the flex channel interleaving. When you get the opertunity, drop another stick in to even it out.
See more info about Ram channels and Flex mode here: https://web.archive.org/web/20081101200235/http://www.intel.com/support/motherboards/desktop/sb/cs-011965.htm
You are running in Flex Mode because you have a total of 4GB in channel 0 and only 2GB in channel 1. The first 2GB of memory in channel 0 is in Dual Mode with the 2GB in channel 1. The additional 2GB in channel 0 is in Flex Mode, that is the same as saying it is running in Single Channel Mode. So, you have 4GB in Dual Channel plus 2GB in Single Channel. It probably doesn’t affect your performance at all but if you add one more 2GB module it will all be in Dual Channel Mode.