I have the same issue, and the solution IS built into the mobo. But my manual did not make it totally clear until I hunted a bit.
My old mobo had six rear panel 3.5mm jacks for audio:
Lime Green for front left / right
Orange for front Middle / Subwoofer
Black for Rear left / right
Pink for Mic In
Light Blue for Line In left / right
Grey for Side left / right (on 7.1 speaker system)
All that matched the plugs on my 5.1 speaker system (nothing to go into Grey), and I was good.
My new mobo has THREE rear jacks: Lime Green, Blue and Pink, like yours. I finally found in my mobo manual that what they do depends on what you tell your mobo audio configuration they will feed to.
IF you use them for a simple 2-channel speaker system, they are used as: Lime Green for front left / right, Blue for Line In left / right, Pink for Mic In. BUT I have a 5.1 system like you!
THEN I found out that, to connect for 5.1 speakers, you do this with the three jacks:
Lime Green for Front left / right
Pink for Front Middle / Subwoofer (orange plug from speakers?)
Blue for Rear left / right (Black plug from speakers?)
For plugging in the mic on my headset, there is NO Mic In on the rear, and I have to use the Pink jack on the FRONT of my computer case.
Check the manual for YOUR mobo for this kind of detail. It may NOT be exactly as my new one is.