The Witcher 3: Hearts of Stone is the first expansion, but canonically, it takes place either in the middle of the base game or at the end. However, if you want superior narrative cohesion, then we'll discuss the best time to start Hearts of Stone in the middle of a playthrough so you know when it makes the most sense to start it, and why we've chosen this specific point in time as opposed to after the credits roll.
You should start Hearts of Stone during the main story while you're still in Novigrad or after the main story. The entirety of the Hearts of Stone DLC takes place in Novigrad, and it's also fairly short when compared to the second expansion, so you should treat it as a small detour from the base game and the quest to find Ciri.
When you hit the Level 30 mark (and preferably before Act 3 and the "Island of Mists"), you should jump into the first mission, "Evil's Soft First Touches," by finding the contract by Olgierd von Erec in the Seven Cats Inn. The XP, Crowns, and rewards for these DLC missions won't break your immersion by making you overpowered when you return to the base game, but it is a fun diversion that also fits in the main story.
Playing through Hearts of Stone before Act 3 of the main game will unlock different (but minor) dialogue options, with most of them just being for fun with no real consequence. It also makes sense, as Geralt would take a contract as a Witcher to prepare for Skellige, only to find himself indebted to a Man of Glass who's clearly not of this world, adding further tension as he tries to fulfill his contract with Gaunter O'Dimm and locate Ciri before the Wild Hunt does.
If you find yourself underleveled, then there's no reason to focus on Hearts of Stone until you complete the main story, as at that point, you should be over Level 30.
While this is how I'd view the Hearts of Stone DLC, it's by no means the only way to experience the expansion. If you want to stick with the base game first, then you should definitely do so! Coming back to Hearts of Stone once you've completed the main story still makes complete narrative sense, as there's truly no "one way" to play Hearts of Stone.
Regardless of your choice, this DLC should be completed before Blood and Wine.
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