Power Macintosh G3 systems are available with either 2MB or 6MB of video memory.
The manual "Setting Up Your Power Macintosh" and the Technical Information booklet say that the system has 2 MB of video memory built into the computer's main logic board and that you can add video memory by installing a Small Outline (SO) Dual Inline Memory Module (DIMM) in a slot on the logic board.
However, if your computer came with 6 MB of video memory, a 4 MB SO-DIMM has already been installed in the expansion slot on the logic board. You cannot add any more video memory.