These three games have only one common feature: they are set in FFG proprietory fantasy setting, Terrinoth (now referred also as World of Mennara). You'll find similar artwork, same Heroes (of course with different conversions of their abilities pertaining to each game) and a lot of focus on combat. The new edition of Dungeonquest has added some complexity to the combat system of the original game, plus more quality and variety to the components. Besides the distintive mechanisms and styles of gameplay, each of these products is highly combat-oriented and mixes it with different degrees of exploration/character advancement.
Trying not to repeat what the others already said, I'll make a summary based on three game aspects that for me are very different in these three games.
Character advancement: low.
You'll find some objects during your quest but the purpose of your Hero is not to improve his/her stats. Just hope to see the light of day and enjoy other players' disgraces.
This is a game based upon exploration, but it's highly random. It's a sort of Dungeon of Death with 75% bad happenings, 5% good findings and 20% sighs of relief (=no effects).
Story elements: low.
It's not a game with a strong story element, it's a beer and bretzels game. Play it for fun: laugh with friends and see who will be the winner, if there will be anyone. It has the advantage of being short in comparison to most fantasy adventure games and it's not an "epic" game.
Character advancement: high
This game is a level-up game. You need to boost your Hero until you're able to beat the endgame, possibly before any opponent does. There's no win without leveling.
In Runebound you travel on a map, following an interesting movement system, you face Challenges in the wilderness and then come back to Towns for healing and gear. The mechanism is always the same but it's sufficiently varied and requires good planning to work well, so we may call it a game based on carefully planned exploration.
Story elements: high.
Flavour text is usually less considered by non-English players, but the mix of map-movement, challenges and Events that affect the entire board gives to Runebound a strong setting and background. If you care to read the text and notice the consistent card themes, you'll see a story that unfolds during your play. This is what makes Runebound shine for me.
Character advancement: medium
There are ways to improve your Hero during a game, but you may finish the game with the same Hero, made stronger by the randomly drawn Treasures. You won't get Treasures or money because you slay monsters, but because your teamwork has bested out the Overlord player's resistance. You got the Chest loot at the right time and worked out a good strategy. In any case, your objective is not to advance the Characters but to complete the Quest objectives without losing too many Hero Lives (represented by Conquest Tokens). You can complete a Quest even with an average but organized party.
You're delving into dungeons but you're not discovering things, having encounters or finding your way. There's not very much to see and there's a lot to slay. There's an Evil Overlord player to best: he has traps, he spawns monsters, he surprises you with tricks. You're not adventuring here, you have to be on guard at every step you take or the game will end in the blink of an eye. I won't consider Descent a game based on exploration; it has a predominant tactical content.
Story elements: medium.
Descent quests have a setting, a plot and a task to accomplish. Unfortunately, most players don't care very much about the story as it doesn't matter to gameplay. Focused on combat tactics and movement, Descent Hero players will not have time for the background as a Runebound player will.