Yes, it is possible to memorise 3 essays in 6 days. As per the above, one essay should not take you more than one day to fully memorise, provided you maintain your focus throughout this process. An important thing to consider that would speed up the memorisation process (which would be advantageous/time-efficient given the fact that you aim to memorise 3 essays in 6 days) is your familiarity and understanding of each essay, which is only possible with an equivalent level of understanding relative to your prescribed texts. This means that the more thorough this understanding is, the faster you will be able to retain all the necessary information. This is something I have personally experienced with The Curious Incident of the Dog in the Night-Time and Kenneth Slessor's poetry when I completed my HSC (English Standard) - thanks to my understanding of the former, I was able to fully memorise it within less than 2 hours (if I recall correctly, it might have even been close to one hour), whereas I needed more time to fully memorise the latter, easily above 2 hours.
I hope this helps!