How much fondant would I need to cover 3 square cakes sizes, 6, 10, 14??
Depends on how thin you roll it and such. However, Wilton has a chart which might be a good starting place.
you can go to satinice.com and find that info ... you can also find it at fondantsource.com just click on the fondant and gumpaste then click on satin ice and you will find it there