I dont know if applying more than 3 years is looked down on. There are people here on SDN who report more than 3 application years.
What is looked down on is reapplying before "fixing" the problems on your application. So, IMO, the biggest question isnt whether you will be spending your last application chance... but, have you improved any supposed deficiencies.
Have you discussed your rejections with ADCOM directors? Do you know what is holding you back? If your GPA is hurting you or if they are unsure you can handle the class load - take a year of classes full time. If its your MCAT, study like crazy... etc. Sounds like you've got the "exposure to med" part though.
Good luck.