If you're looking to go as cheap as possible, here's a basic run down of the bare bones you'll need. Midwest might have similar items, but I got a lot of my stuff at Kegconnection so I can show you what I'm talking about. Kegconnection is also great if you're looking at a kit, most of them they will customize to what you want, so you could even get ahold of them, tell you what you want to do and what you've already got, and they'll get you the rest.
First, you'll have to decide if you'll ever want commercial kegs. If you do, there are
conversion kits that can switch between sankey and corny using the MFLs described above. Otherwise, you can simply cut the tubing right at the sankey connector and attach the corny disconnects. It looks like you've already got one gas (and I'm assuming one liquid), so you'd need two more gas and two more liquid disconnects for your three kegs.
For the gas side, a T is going to be the easiest way to split the two current gas lines into three. Just cut the gas line about a foot past the regulator, install the T and a new line, and you're good to go. If you make sure to never get liquid inside the gas lines (ie don't fill up past the gas outlet) checkvalves might not be necessary. However, if you want to be safe, checkvalves are a great way to protect everything, and you may want to check the splitter on the regulator to see if it has checkvalves already.
Now the liquid side, the cheapest route is going to be a picnic tap for the third keg. So long as you make sure the tap doesn't get accidentally turned on (maybe something to hang it out of the way), there's nothing wrong with a picnic tap. If you do decide to go with a tap, you'll need the
shank for through the door, the connectors to attach the beer line to the shank (
tail piece,
hex nut, and
washer), and a
faucet. They also make
shanks with a barbed nipple welded to the end so you can skip the connectors, but you'd have to compare prices. You can also find kits that come with the faucet/shank/connectors all in one.
Add a third keg, and you'd be good to go! That being said, if you've got a little extra funds and the inclination, there are a few places you could upgrade that would make your life easier. For starters, since this is a hand-me-down kegerator, I'd probably suggest switching out all the line for new ones. You'll already be buying some to set up the third, so get a little extra. Gas line is less important, but you'll feel better with nice, new beer line. Its suggested to start with 10 feet of 3/16 beer line for each tap, but there are balancing equations if you want to take a crack at them.
Also, if you decide to go with a faucet, take a look at the Perlicks. The cheaper standard faucets are great for bars where they're constantly used, but I know from experience that if you don't use them daily, they'll stick up on you real quick. If nothing else, start out with a picnic tap for the third keg and see how the other two treat you (assuming they're not perlicks) before buying a cheaper faucet. Otherwise you'll just be paying more to upgrade in the future, like I did!
Getting away from those, there are a bunch of other upgrades you can do that don't necessarily need to be done. Instead of chrome you can do stainless everything (shanks/faucets). Rather than the regulator you've got, if you want to do multiple pressures you can get
additional bodies on a regulator. If you're gonna have three kegs (or four) on tap, you'll probably find that you want to have more than that sitting around building up a pipeline. Sometimes you can find deals on bundles of four kegs, which might be an option. Larger CO2 tank will mean less trips to get it filled. I could go on and on, but hopefully that gives you a start.
Oh, and I bet some
goo-be-gone will do wonders for those stickers
.
