1) Are they powered speakers? If not, you'll need an amp
2) Are they RCA or phono jack? If RCA, you can connect the left and right using an RCA to TRS converter, and if phono you'll need a dual mono to TRS
As for the sub, you'll need to either make your own low pass filter and connect it to left/right, or find a proper amp that has a sub output.
Most likely it'll just be easier to just buy decent powered speakers.