well if the fans still have the 4 pin adapters on them to hook them into your computer power supply you can simply just hook them all together, then wire the adapter output to 2 of the holes that carry power (the other 2 don't do squat).
If you want to work with bare wires you can hook them up either in series or in parallel
Parallel will have all the reds connected, and all the whites connected, then hook that up to positive and negative of the power adapter
in series you hook them head to tail, ie power - red -- fan -- white -- red --fan2--white--red-- fan3---white-- other end of power.
I'd hook them in series because you can always run them at a lower voltage (they go slower) if for whatever reason in parallel all 3 draw more current than your power supply is putting out you could be in trouble. If they already have plugs then don't worry just plug them together (i think they're wired for series that way).
Now you can't really hook up the power wrong, if you hook it up one way they'll spin a particular direction, if you hook them the other way they'll spin the other direction, Which way you want them to spin may not matter, I doubt they put any sort of special slopes to the fans to be more efficient. Just make sure the side that blows air out is pointing towards the water
