Some Galaxy S models (like mine) have a corrupt boot loader. This fix implements a secondary boot loader which allows you to successfully enter Recovery and Download modes using the 3-button methods. As hilton already mentioned, it is an absolute necessity to get it working. For now, you can enter Recovery mode using ADB (Android Debugging Bridge), which is part of the Android SDK (downloadable for free from the Android website). ADB is a command-line utility which allows you to reboot your phone (which should be connected to your computer via USB, with the USB debugging setting on your phone enabled) to Recovery or Download modes. You'll need this utility to implement the 3BR fix, so get downloading already!
EDIT: Ah, prof818 beat me to it. Slow, elaborate typing FTL.