Ok guys. What is going on???!!!
I uninstalled Windows 7 from my laptop, then tried to activate the product key on my new Ivy build, didn't work. Called M$ who told me that I can activate only one computer and I must uninstall both previous computers. So I did exact that.
Than I was finally able to activate my new computer. So after that I decided to try my luck with the same key on the old computers, and both of them WORKED! Now all three computers are activated on the same product key. And it's not even a family pack. All three systems are activated with unique Product ID.
Am I lucky or is M$ activation system flawed?