I've never installed an SSD to a G3 iMac but have done to a G4 iMac.
I believe I used a StarTech IDE 40pin PATA to Sata 2.5 adapter which cost about 20 euros (US$ 22) the quality of which was good. You can find much cheaper, but the soldering quality is not top rate and lots of flux residue is left behind on the board; I tried one after thorough cleaning the board with isopropyl alcohol and it functioned ok with a Sandisk SSD.
HERE's the cheapo version, and the more expensive
Startech adapter.