Dual Channel is not always the same on every board. Some boards will allow you to have a dual channel and single channel active at the same time. If your mobo allows it, you can run the pair in dc mode, while the single module will run in sc mode. If you opt for the dc modules plus 2 additional modules, you might as well just go for dual channel for both pairs.
How can I find out if my motherboard (the Asus M4A88TD-M/USB3) can run dual channel + single channel?
Best Regards, -Josh