Give MacTubes a try - it works wonders on my G4s and G5s. I'm not sure about a G3, however, but removing Flash can't hurt. Don't expect any Flash content to be a hot performer, though - unless, maybe, it's rather old.
Also, if you want to play offline video, try to find the last G3-compatible version of VLC.
I'd suggest you look into a new machine - unfortunately, G3s just don't have the muscle to do everything well nowadays. Getting a bit of RAM might help, but I believe those machines max out at 640MB.
Don't pour too much money into it on anything that can't be transferred to a new machine. Maybe $20-30 for a bit of a boost would be good, but much more than that and you're getting a bit iffy. (Knowing the prices of older RAM, I dunno if you're going to get a really good deal. $35 for my 1GB module in my iBook was a lucky find - it's usually far more!)