Windows 10 activation

I was able to install Windows 7 on three computers with my Windows 7 key code. Will I be able to update all three to Windows 10?

Of course.
Updating to Windows 10 is the easy bit... supposedly!

You may have trouble activating the three computers if you used the same key to install three copies of Windows 7.

When Windows 7 first came out they sold a 'family pack' of three licences for $150.
But they came with three activation keys.

I wouldn't bet on activating all three of your computers to Windows 10, but there is anecdotal evidence to suggest that you may get lucky.

If you DO decide to go ahead with this dastardly attempt, do each computer one at a time and make sure that each version of the installation is 'activated.'

(The first computer you want to upgrade is the one you most want to have a legal copy of Windows 10 on it, as the first installation process you do is the one that is most likely to succeed in activating.)

On your first computer you should upgrade Windows 7 to the current version of Windows 10 and activate it with the key you used for the Windows 7 installation.
(Keep all your files, etc for this one.)
Then do a clean install of Windows 10 on top of the Windows 10 upgrade and see if that gets activated automatically.
(Reformat and clean install.)
(Do NOT enter a key for this final installation.)

Then wash, rinse and repeat for your other two computers and hope you get lucky!

Good luck.
Let us know how you get on.

Mines still giving me activation server determined the Key is blocked and i upgraded from windows 8.1 Pro to windows 10 Pro but still its not activated