Totally different animals. The FAL is a gas operated rifle based on a machined solid steel receiver. The CETME/G3 rifles are based on a sheetmetal stamping receiver and are roller delayed blowback operated. Nothing interchanges except the ammunition!
The CETME (original Spanish rifle) and the G3 (later "product improved" German made and licensed) are two versions of the same basic design, but major parts are different between them. They both can be built on the same receiver shell, the only difference between the current receivers are the welded on rear sight bases. The trunnions are different as well, so if building a CETME make sure the receiver comes without a trunnion.
The FAL was probably the most popular .308 rifle ever produced, adopted by many many countries in one form or another. Right now about the only kits commonly available from vendors are the Imbel made ones. The other variants are available on the various boards' for sale ads as well as Gunbroker, but you are talking a lot more $$$ on those. There is currently a receiver shortage on these, only DSA seems to have much in stock and thery are the most expensive out there.