I have already answered this question, but I will duplicate it with a few minor clarifications.
A complete list of hotkeys for the original Heroes 3 does not exist; in fact, there are very many of them across various menus. The main hotkeys are listed in the manual for the original RoE game (ignore the link formatting): http://heroes3wog.net/download/[Heroes%203]%20Restoration%20of%20Erathia%20Manual.pdf
Hotkeys that seem to match those in the manual are also listed here: https://homm3sod.ru/keys/
For the HD mod, hotkeys are described on its website: https://sites.google.com/site/heroes3hd/rus/description/extended-ui
Specifically in HotA, the following hotkeys have been added:
Adventure Map:
— While holding Alt, you can view the path length (in movement units) of the current hero to different map cells (displayed in the bottom line and via right-click).
— While holding Shift, you can select a hero by clicking on them on the map (instead of ordering another selected hero to go exchange with them).
City Window:
— E — hotkey for exchanging between heroes.
Combat:
— G — hotkey for Fairy Dragon spellcasting and the area shot of Magogs, Liches, and Powerful Liches. The original F hotkey for fairy dragons is no longer active.
— Alt — hotkey for switching stack actions (melee attack/shooting, return/do not return after attacking, attack/spellcasting, move/spellcasting).
Thank you.