Mass Effect 3 won't install after buying the Trilogy

Firstly I'd like to apologise as this may be a frequently asked question but i do need an updated solution.

I just bought the Mass Effect trilogy for PC and I installed Mass Effect 1&2 without problems but when i tired 3, it told me i had to activate it using a product key. So i use the product key they gave me and it said it is already in use and downloading with the internet is really not my 1st option because of a  slow connection. Please can somebody help me