Urgent Advice Please: Level 7 admission, visa expiring soon

I came in Uk in July 2011 on a Level 7 course: (Now I have been in UK for three years)

Here is my detailed visa history:

Entry Clearance (First Visa)- 21-6-2011 to 18-6-2012

First Extension 06-10-2012 to 16-01-2014 (Level 7 Course)

2nd Extension 18-06-2013 to 30-10-2014 (Level 7 Course)

Now my question is, Am I allowed for one more extension in Level 7 course? I have heard that you can only stay in Level 7 for 3 years
Is my allowed period over for me? All my three colleges were highly trusted but they all disappeared from the UKBA list when I was on the verge of completing my course. Now I have offers from some colleges for level 7 course admission, but I want to be sure that I can go for another extension in level 7 or not? Please guide me in this regard and if possible direct me to the ukba rule regarding this. No body is guiding me in the right direction. Colleges want to sell their CAS and I dont want to get refused by ukba and lose my money just on the basis of level 7 allowed period.


Regards

Asim