I am pretty sure you can just unplug the old fan from the PCB on of the power supply. I am sure that's how I did mine. But, since you cut yours already, just spread the wires out on both halves ( like a fan) then, twist both ends together, end to end, and apply solder. Heatshrink tubing afterwards.
Like this:
https://www.instructables.com/id/Soldering-Tutorial-Inline-Splicing/No, the polarity of the wires has to be matched red to red, black to black. The yellow wire is for speed control, but, that PSU isn't capable of controlling speed, it's all or none. That's not what determines what way the fan operates. The way you place it back into the case is how it directs flow. My A3000 blows air OUT. It sucks cool air in from the grates on the front/underside of the faceplate of the case, and, from the perforations in the backplate on the side of the daughterboard where expansion cards are plugged.
