I've tried simply pressing and holding "C" after turning on the switch. I've tried Command-C, Alt-C, Command-Alt-C, Command-Ctrl-Alt-C : all to no avail, the hard disk just kicks in and boots OS9.2.
I tried Command-O-F and that gives me an error message on my monitor screen saying "SIGNAL OUT OF BOUNDS".
I even tried installing the "Mandrake Linux Install.sit" file, from CD, or copying to disk and then starting. In both cases, I get the error message "The application Mandrake Linux Install.sit can not be opened because an error of type -39 has occurred".
The beige G3 is an "Old World" machine, which basically means it has a big ROM chip containing half of Mac OS, and some older version of the boot code that doesn't support booting non-Mac-OS directly. You have to use BootX to start Linux out of Mac OS. BootX is contained in the "Mandrake Linux Install.sit" archive. Due to a misconfiguration of the Linux program that created the CD, this file is marked as being an application, although it isn't - that's why you get the error message when you doubleclick it. Just drop it on StuffIt Expander to expand it.
The AppleScript applet that should automate the BootX setup has some flaws too. Try it, but don't be surprised if it doesn't work - I recommend you configuring BootX by hand. I don't know the necessary options offhand, but if you have problems just ask and I'll look it up.
-Christian
