To install patch you should copy file (right-click and SAVE AS) to your \DATA\ folder (example: c:\heroes3\DATA\). DO NOT change file name! To un-install patch - delete file from \DATA\. Remember: A-patch and E-patch must be at both players, else they will work only for patch owner! B-patch and C-patch can be at host PC, will work for both. Patches work independantly in any combination. Attention: check filesize after download, because website is slow! Correct sizes mentioned above.
Disclaimer: D-patches were made not by me, i found them in internet.