When I launch Heroes 3 with HD mod, the game opens but it's not HD. The game looks exactly the same whether HD+ is clicked on or off.

Any idea what is wrong?

There are more options that just checking the HD+ box.

Yes, some of them make graphs slightly better. But none of them makes it even close to the HD version of the game available of Steam. Or have I misunderstood something? I thought the mod's point is to make the game look like the HD version available elsewhere?

Hard to say. I don't know how you want the game to look. Some prefer ugly stretched images, while other prefer correct aspect ratio and black borders.

Ubisofts mobile backport look terrible on PC. What you exactly mean by HD? they redraw things to worse. Although resolution are better.

Heroes 3 HD+ mod keep good draw with modern resolution. Shouldnt be black bars. But im notnsure how HD+ work out of the box after it start to use Heroes3GL as render. And im switch to HeroesGL entirely. For HeroesGL there is less settings but they need to be set. By default its keep 4:3 as i remember.

So clearify which HD mod you use? I mean you sure it is Heroes3HD+ mod (and not Heroes(3)GL wrapper mod). I guess someone here already get such confusions before creating threads about how to configure the mod.
We now have few sticky threads for differ mods.There is also HOTA which include HD+ which include HeroesGL (and 3 threads here for them). Confusing, isn't?

Trying to play this on a Galaxy Tab A9+...I downloaded winlator and it won't allow it to launch. Says it needs .net software. Help!!!

I did answer that in other thread keep it under clear GOG version. Here is muly answer for modded onem This thread is about Heores3 HD Plus mod. I suppose it harder to emulate. It have game+mod pre- Launcher with own system requirements and OS redists dependencies.

If yylu want modded game way better is HeroesGL wrapper-mod by Verok. It is better as compatibility wrapper (HDPlus mod are more than just HD including gameplay changes). It make better HD/wide support. It is faster.
And it have better compatibilities with non-Windows emulators.

Eo first try to get working clear GOG version (and pre-extracted one). Only then install HeroesGL. Again - you need PC(-Win/-Lin) to install and configure game with HeroesGL (pre setup config for it too).

I doubt you be able to get Heroes 3 HD Plus mod working there.

If it is Android OS device you can try Heroes 3 HD Edition official. Restoration of Erathia only. But i hear someone try to use it as engine to port Heroes3Complete too. Did modders able to do that?

Maybe there is un-official remake engine like FHeroes2 for Heroes 2.