As per the title.
The installation goes to 2% then fail.

I downloaded the installation files and installed the game from them.
Then Galaxy wanted to do some updates.
This update failed few times but finally succeeded.
It's a bit messy.
I do not think/know wheter it is related to Homm 3 or a surge in service requests due to the sales, but improvements are to be considered there.

Try the offline installer and see if it holds up?