no it's not like that because it depends from how many players you have in defense -midfield and attack.
It's not 3 equal parts.
You can only calculate it like that if u use a flat formation like 442 if u use a formation that has amc/amr/aml or dmc/dmr/dml like a 451v formation the quality of ur dmc will be used to calculate ur overall defense quality and his quality will also be used to calculate ur midfield quality.thesame thing with ur attacking midfielders.this double count will give u the wrong information if u use it to calculate total quality, so the system calculates ur total quality by adding the individual quality of all ur players and dividing by 11