I am due on this Thursday the 15th Sep and have been having some cramps and backache not major and they come and go but feel totally different to period cramps my boobs are tender as well and feel heavy and I have increased clear discharge (sorry tmi) I was wondering if anybody knew if it's possible to implant 3 days before period is due and that could explain cramping
Yes it is possible. If you go to a site called countdown to pregnancy there is an implantation calculator, if you put in your ovulation date it will tell you the likely days you could implant.
Implantation days are from day 6 to 12 following ovulation (usually 8/9/10) so if you have a 12 day luteal phase and implant on day 6, you could in theory get a BFP 8 days early.