This is not possible without hardware modification. The iMac G3 does not have a video input port; some (but not all) models have a VGA output port, but this cannot be used for input. Even if you could make it work, the maximum supported resolution of 1024x768 would probably be unusable with modern applications, which are likely to assume typical modern display resolutions (ie at least 1920x1080, being the most common resolution on desktop Macs, or even higher for modern MacBooks with "retina" displays).