Are the last three trays of Invisalign treatment necessary and essential?

That's correct, the last 3 aligners are optional, they are called overcorrection trays that Invisalign provides for every case.  They may or may not be necessary, it just depends on the individual.  The main purpose of these trays is to close any residual spaces or gaps between the teeth.  You can determine this on your own if you feel little to no resistance with floss as you go up and down.  If all the spaces are closed, you have good contacts between all your teeth, and you and your dentist/orthodontist are happy with the result, these aligners will not be necessary and the treatment is complete.  Hope this helps, good luck!