Is it worth it? I mean from a story / fun standpoint? While I generally like going back to the roots and observing what it's like, too often I find that it really hasn't aged well (saw this with Adventures of Mana, for example). I haven't dug in DQ11 yet because I still have JRPGs to finish beforehand (Secret of Mana, delaying this because the platinum grind is so awful; Ys 8, delaying this because I don't want it to end; TWEWY, actually playing this one and only delaying this to collect SDPP), but I saw DQ 1-3 in the E-Shop and feel kinda tempted.
They are definitely worth it in my opinion, at least 1 and 3.
1 isn't a long game, it took me about 10 hours to finish with no guide. The story isn't anything special, but it's still charming in a classic kind of way. But especially with the new quick save feature in this whole trilogy, it makes them so much better to go through instead of using the old archaic save system. And the gameplay is a solid old school jrpg affair as well.
Now Dragon Quest 2. I enjoyed it, I really did, but it's the hardest of the three for me to recommend. It has several very obtuse moments with just what I consider bad game design in how you're supposed to figure out how to progress. It isn't a constant issue, but as someone who never looks anything up in games, I had to look up how to progress in this game 2 different times. This may not be as big a deal for other people, but it was a huge deal to me. But other than that, it's an improvement over 1 in every other way. You now have 3 party members instead of one, the world size is almost quadrupled (with the ENTIRE world map of the first game being present and taking up only a corner of this map). But like the first game, the story is fine, it's pretty much the same story but on a little bit of a bigger scale.
Dragon Quest 3 is the best game of the original trilogy by far. I'm not done with it yet, but I can say with confidence that it's the best. It introduces a job system to the Dragon Quest series, and the job system is sooooo addicting. Say you make one of your party members a fighter. Once they hit level 20, they can change jobs if they want. So maybe you'll change them to a thief. Their stats from when they were a fighter don't go back to zero, but are cut in half. So now you have a thief with the stats of a level 10 fighter. Now get your thief to 20, and it's agility has went up tremendously. You can now change jobs again, and YOU CAN LITERALLY GO BACK TO LEVEL 1 FIGHTER, BUT NOW YOUR STATS ARE HALVED AGAIN. You can keep doing this if you're so inclined, and you could end up with a super fast fighter or a super strong thief. This is just one example though, you could cycle a character between as many jobs as possible to learn all of the skills, or stick to one job the whole game to level as high as possible as quick as possible, any option is viable. Each character also has a personality trait, which affects just how their stats go up with each level, it's a very deep system.
3's world is also the best of the three, and it's loosely based on our real world map, and the locations reflect that, giving it an odd familiarity and making it a little different and more interesting than usual in jrpg's to explore.