Hello

I have the complete Heroes of Might & Magic 3 game, but the image is not adapting correctly to Fullscreen.

Is there a way to change screen size?

I tried the HD mod but it crashes everytime I start the Restoration of Erathia campaign.

You don't exactly give us anything to work with. To start, what OS are you using, and if there's more than one installer, which one did you download?

Sorry I don't know a lot about this kind of pproblems.

What is OS in this case?

Operating System so is it Windows, Mac or Linux and then which version i.e. Windows 10 or 11

What Operating System are you using, and if it's Windows, what version? And, while I'm thinking of it, if you're on Linux, what distro?

Okay !

I am on Windows 10 Family version.

Keep Aspect ratio + forcing checkbox (if there is such checkbox) in video driver. Maybe in display OSD settings or driver as well.

I don't know where to find any of these things.

Lower to anything your desktop resolution. Look to video driver settings. If you have Intel+Nvidia NV can took of from Intel driver. Look to both anyway.

http://www.damirscorner.com/blog/posts/img/AmdCatalystControlCenter.png

If current Desktop resolution is differ aspect to display aspect ration - black bars will appear. That mean it work properly now. You can set back any resolution you prefer for Desktop.

Game itself have only one (800x600 I guess) resolution and should be scaled to 1440x1080 on 1080p displays. To make it wide-screen if this is all you need - install mod. HeroesGL (Heroes3GL variant) wrapper mod by Verok. It have also some else minor features.

To more features including losing some gameplay aspects - Heroes3HD+ mod.

For both or any else wrappers and mods you should check and remove compatibilities, use Verok's AppCompatChecker tool.

Or keep it original 4:3 1440x.

I only have Geforce.

I don't know where to modificate ratio. I don't find HeroesGL by verok (only for Heroes4) and the Heroes3HD mods makes my game crashes everytime.

It should be somewhere. If there is no in Nvidia CP, look to Intel. It probably available from from context menu by Right Mouse click on Desktop - aspect ratio - keep aspect ratio. Or in both.

If you read the post, you'll see that alien30 says that he has a Geoforce card. Why would he have nVidia drivers?

First he should lower resolution on desktop. That make aspect ratio options available. In Intel, Nvidia, or AMD drivers. In two or only in one of them. He should look better. Whatever card(s) he have. Intel options accessible from Desktop by RMB click.

As for Heroes3GL, although latest builds probably not saved (Verok have many minor updates for his wrappers/patches keepsake version number), one of latest should be somewhere. PCGW, ModDb, WOG Heroes website (related to WoG?) or some Russian... old-games RU maybe...

Internet Web Archive saved his mods, but very outdated version (BTW github pages pre-compiled and even source codes All are for outdated builds).
There is separate uploads on Internet Archive , but same as most sites it is one of older version but not most up to date.
And some sites for some wrappers have very older version saved. Like DisciplesGL v.1.89 two years outdated while the latest is 1.90 from '21 (with Feb '22 fixes). But 1.89 is good to go too. So you need find one of latest, and you can find it somewhere.

(I'm keep latest ones only for Blood Omen and Disciples)

Or you can Donate to Verok's patreon.

UPD: May '21 builds of all HeroesGL (and few older from '19) in this post
http://www.old-games.ru/forum/threads/serija-igr-heroes-of-might-and-magic-gl-wrapper-patches.80503/page-33#post-1799839
I guess they are not latest. All else links, from Eng internet, you find yourself.

Or this post
http://www.old-games.ru/forum/threads/serija-igr-heroes-of-might-and-magic-gl-wrapper-patches.80503/page-33#post-1799769

And Ofc our local thread for HeroesGL on last posts there is many links.

Heroes 4 in GOG have own wrapper - rename _ddraw.DLL file. Heroes 1/2 are for Windows versions (from got provided disk images or localised installers some based on win version, or separate Win installer Heroes2).