downloading the third install rather than using the disk

I have looked but can not find an adequete answer to this.

 

I have Norton Internet security, I have used two of the installs from the disk but I now have a third comp' I want to install onto. I have lost the disk.

 

Is it possible to download the the third install? (accepting of coarse that this is the third and final use of the disk.)

 

Frankly the website is hard going when it comes to navigation and I just want to find an answer.