Bicycle Mechanics - Converting 3-speed to single speed

I'm just getting into bike repair, and had a question about a thrift store bike I'm fixing up. It's an Iverson 3-speed cruiser with an internal hub (an old Shimano I believe) and normal brakes. I'm trying to make it into a single speed to ride around my city.
Currently, I have an end nut on the end of my axle to hold the pin in and engage the gear, but this makes for a very uncomfortable ride. I was wondering how else I could make it a single speed. I was thinking:
1) Can I use the same back wheel I currently have and put a BMX-style single-speed freewheel on it?
2) Is there any way to turn this into a single speed internal hub? I have access to a bike co-op near me with tons of used parts, and I've read that single speed cruisers exist.
Thanks a lot. I'm welcoming any ideas at this point.

The three speed would be useful. They worked great. Get the three speed working again with parts from the co-op.

Maybe all you need is a shifter,cable and the lever that threads onto the axle.There may be nothing wrong with the hub.Sometimes it's really nice to have a selection of gears at your disposal.Are there any hills where you are?
If the end nut you have on it is keeping it in too hard of a gear(3rd),maybe you can screw it on further to get the rod into the position for 2nd or even 1st.

Just tie off the gear cable so that the hub is securely in second gear -- instant one-speed!

You could set the 3 speed so that it stays locked in second gear which would be a gearing in the high sixties but you'd be paying a fairly high weight penalty for that hub and wheel.

I'd set up the 3 speed so it works or fit a different wheel with an SS freewheel.

An ss freewheel will not thread on to the Shimano hub as it uses a spring clip to retain it's cog and is not threaded on.

The problem I've been having is that the end nut simply isn't engaging the gear most of the time. It gets loose and even when it's tighetened it doesn't perform well. I'm considering tying off the cable and just keeping it in second gear, it sounds like a pretty simple way to make it a single speed without buying a new back wheel.

I've considered trying to make all 3 gears functional, but thus far I haven't been sucessful and I'm afraid to take apart the hub. There are far too many parts in there for me to ever put that back together.
Thanks for your help, everyone.

"My basic recommendation is to avoid the Shimano 333 hub (http://sheldonbrown.com/shimano333.html). Generally it was only ever spec'ed in low-end bikes that probably are not worth fixing anyway." ~Sheldon Brown

Check this link......

http://www.bikeforums.net/showthread.php?t=393362 (http://www.bikeforums.net/showthread.php?t=393362)