I have wide screen monitor. Is Heroes 3 wide screen?

The vanilla version isn't, and AFAIK there is no way to change the resolution right off the bat.
However, HoMM3HD is an awesome awesome mod that lets you choose any resolution. How well it works with widescreen, I'm not sure since I've always played it on 4:3.

It works perfect on a 16:10 or 16:9 screen, even adds some nice interface features if you want to.

Yeah, I've played it at 1360x768 on my 1080p monitor using that very awesome piece of software. Any higher and the icons and text become way too small.

I play HoMM3 in full 1280 x 800 in WINE using this mod, and it works great. I'd think this would work with Heroes Chronicles too but I'm not sure how.

Funnily enough, I only tried it out for the first time last night. It's as terrific as these people say, though I think the lowest widescreen resolution available is the best. Teeny tiny units and fonts hurt my eyes. ;)