Hi,
that kind of editing is what I'm trying to avoid as several people will be using these files.
I finally got it working by changing my formulas to use the indirect function. This way missing sheets will not come up as errors and cells will be updated automatically when the data is added.

changed:
=AVERAGE(Folha1!$C$3:$D$3,Folha2!$C$3:$D$3,Folha3!$C$3:$D$3)
to:
=AVERAGE(INDIRECT("Folha1!$C$3:$D$3"),INDIRECT("Folha2!$C$3:$D$3"),INDIRECT("Folha3!$C$3:$D$3"))

thanks,
Carlos