Hi Vamsi,
As per our expert caatano's suggestion, create a BOM for main Material (FG1) with the component of raw material requirement and also mention that another FG2 as requirement with negative(-) qty. When ever you do confirmation for FG1 production order, FG2 will generate automatically. If the third output FG3 (wastage/scrap) having some resale value means that also mention as an requirement of main FG's (FG1) BOM with negative qty. What ever the material's qty are maintained with negative mark in BOM, those material's qty will be generate automatically while doing production confirmation. If the wastage (FG3) don't have resale value then you can define that material as co-product.
Thanks