Using your current equipment, you have two options for connecting the MR4 and subwoofer:
1. Use a 3.5mm to RCA adapter plus two RCA Y splitter cables to connect both MR4 speakers and the subwoofer to a single Line out port on your motherboard's rear I/O
Basically, these three connected together:
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2. Use your motherboard's 7.1 surround functionality to connect the two MR4s to one 3.5mm output jack and the subwoofer to another jack.
For this, you will need to install the correct audio driver, afterwards there should be a pop-up whenever you plug in a 3.5mm plug, asking what kind of device you just plugged in.
Option 1 has the advantage that no matter what content you'll listen to, the subwoofer will always play and provide deep bass response.
Option 2 has the advantage that, once configured, you'll be able to watch movies and TV shows with the LFE channel being correctly routed to the sub.