There's a popular recipe floating around which only uses the glycerin, but as far as I know you want to use all three of them because each of them has something weird that will happen to the soap if you use too much of it by itself, although I cant remember off the top of my head what each side effect is right now, I'm sure someone else will chime in with more information.
Do not use only glycerin. I tried it! Unless you live in the arid dessert, you'll end up with a pool of soap in a few hours. The glycerin pulls so much water out of the air that it dissolves your soap. I've had good luck getting clear soap when using glycerin, propylene glycol, and sugar water. You must use a recipe high in stearic acid and lauric/myristic acid to get good hardness and bubbles.
^^^What Galaxy said. Just glycerin will Not work. There is a video posted in one of the threads here, but I do not have time to find it. One warning while it is fun to make I find the melt point so low I cannot get it to pour without winkles plus the fact it does not save money to make your own. I have several different recipes I have tried and while nice they are not as nice as a good purchased base for pouring decorative. We use one of mine for a lovely facial bar poured in a oval cavity mold. Link for the video http://www.soapmakingforum.com/showthread.php?t=36235&highlight=melt+pour+scratch