I think you are mixing up installed memory (RAM) with available disk space. You need about 5GB of available disk space to install the app, but this has nothing to do with RAM.
There is the possibility to install content on an external drive, but this is a HACK! I would suggest not to try, rather clean up your drive, if that is the issue. There are tools to find the biggest memory pigs.
If you have the space to download and install one set of additional content at a time then you could follow the guidance the thread linked to below to move content off you main hard disk to an external disk - however I agree with Blueberry in that it sounds as if there's a mix up here between RAM and HDD (unless you mean to say that you only have 3GB of HDD remaining, in which case you probably do need to think about clearing it up soon!).
I should also say that loading external content off a USB flash drive will probably be quite a slow process, especially for larger sample instruments like the grand pianos, so brace yourself...