it seems a stupid question but the documentation of Magic Bullet LUT Buddy says it supports exporting LUTs from Final Cut 7. However the documentation only shows how to do that from After Effects and then import the LUT into FCP. I have not found any description of exporting Effects (e.g. achieved using 3-Way Color Corrector) as a LUT using LUT buddy. I have found postings by people who claim to use it for that but I have not found a menu entry or anything like that in FCP 7.
Found it. In case anyone else doesn't find it obvious either, you have to add the "LUT Buddy effects" to your clip as the first effect and set the "Action" to "Draw Pattern" (you will see the pattern on top of your source clip) and then add it once again as the last effect and set "Action" to "Read Pattern", which enables the "Export LUT..." button.