The A/V personality card that comes with some Power Macintosh G3 configurations can only mirror the built-in video port.
If you wish to add a second monitor (or TV) and use it as an extended desktop, that can be accomplished by the addition of a third-party PCI video out card.
Notes:
1) It is not necessary for the Power Macintosh G3 to have 4 MB SGRAM to do video mirroring. You can attach a TV or second monitor to the RCA video out jacks of the A/V Personality card and do video mirroring with only 2 MB SGRAM.
2) Power Macintosh G3 All-In-One models will also support video mirroring from the DB-15 port on the back of the computer. This port does not support extended desktop viewing.