Get 5, I'd say. It's mostly 3 with some good stuff added in and better graphics. Make sure to get the expansions, though, they make it really shine.
Yeah, 3 could be overwhelming to a new person, I agree. But it's not too hard when you get the hang of it. And all HoMM games work basically the same way.
- You have your turn.
- In a turn you can basically do stuff.
- You can build one thing in each town. Assuming you have the resources, that is, I'll assume you do. You can build stuff and use them immediately. You'll have to wait to build again.
- Your heroes can move. Each has a set of movement points(the green bar next to the hero's portrait) which, when depleted, means he can no longer act. Movement points are used for...movement (shocking, I know), but picking up resources also counts, even if they are right next to the hero, same goes for visiting buildings and so on. Basically, any time you double click somewhere, uses movement. Some artefacts, skills, terrain and effects can modify the movement but I won't go into detail. Clicking somewhere once, shows where the hero will move through and if they'll reach the destination or not. If it's really far away, it may take them more than one turn to get there - the cursor would say (it may show 2, which is two turns, 3, for three turns and 4+ for anything above).
- Your heroes can cast adventure map spells. If they have any and the mana to fuel them. The mana is the blue bar on the left of the hero portrait, or also the number under the scroll in the bottom left panel when the hero is selected). Adventure map spells include stuff like View Air and View Earth which give you more information about the map, or Fly and Dimension Door which enhance your movement.
- You can fight. You can attack wandering monsters (who don't actually wander around, but still...) or enemy heroes.
- If you don't have anything else to do (or don't want to), you can end your turn (the hourglass button on the right or the e key) and all other people/computers have theirs. Once everybody is done, a new day begins and you repeat it all over again.
- every turn you get resources from any sources you have (mines, towns) and each new week, you get a new population in your town, so you can buy more creatures.
So that's basically how the game plays. But there is also combat which, I must say, is pretty similar.
- You get turns again
- Monsters go in order of their speed from highest to lowest (so, gnoll with 4 speed will act before a dwarf with 3 speed).
- monsters can move, attack, wait and defend.
- movement is simple - click anywhere in the shaded area.
- If you can't see the shaded area, click on the computer in the lower left and tick "Movement Shadow". While you're at it, click "All Statistics" (I'll tell you why in a moment) and see if "View Hex Grid" helps you (I like it, while other people seem to find it distracting).
- to attack, just click on the creature you want to attack. If it's in range of melee units, the cursor will change into a sword when you hover over them. For shooters, they can just shoot, regardless where the enemy is.
- to wait, press the hourglass button in the lower right or hit w. This would defer the creature's action for later. Useful if they can't do anything at the moment. Each creature can wait only once in a turn
- to defend, click the shield in the lower right or press d or Space. Defending skips the creature's turn but gives them a defensive bonus.
- once each creature has had their go (either moved, attacked or defended), the turn ends and a new one begins.
- Your hero can do stuff only when one of your creatures is about to act. They can cast one spell per turn, assuming you have the mana. Press c or click on the book in the lower right. Alternatively, you can surrender (to enemy heroes) or flee (from anybody) if you think the odds are against you. You'll have to buy that hero again from the tavern in town.
Well, I think that's a besic overview of how the game plays. Well, the combat section is really about 3, but still, the others aren't too different (well, 5 is slightly different).
Finally, your town. As I said, you can build stuff in them. Click on the buildings to use them or to view information (if they give passive bonuses, such as resources per day). A few buildings stand out and need mention:
- the Hall. Usually the biggest building that looks...well, sort of normal. Click on it to build stuff. Stuff in green, you can build, stuff in yellow you have already, stuff in read you can't right now - either because a lack of resources or because you need to build something beforehand (say, to build a City Hall, you'll need a marketplace, blacksmith and magic guild level 1).
- the fort - the other biggest building that looks like a castle or similar. You can buy all your creatures from there (as opposed to clicking on each of their dwellings).
- the tavern - this one varies from town to town. But you can buy new heroes from there.
In Heroes 5, you get buttons for each, so you don't have to click on each building. It's really better, IMO.
And that's really it, I guess. The rest comes from play. Try playing with cheats on, so you have less to worry about and more time to understand how things flow. Also, a good beginner map would be Good to Go - you start off with all (or most) of your buildings already built, so you can just focus on the rest of the game.
Also, a good starting faction is Castle - they are the human guys and are the best overall - no huge weaknesses. And you get two shooters, which is really nice.
EDIT: I realised I forgot to mention, why you'd want All Statistics in combat - simple - if you hover over another creature before you attack it, you'll see how much damage you'll do to tham at the bottom of the screen. It's really useful.