How much content do the three dlcs add?

[Neffew]

just curious before buying

[Gatebase]

On paper each adds 3 biomes as alternatives to existing biomes, roughly 3-5 new enemies and 1 new boss, plus around 10 new items. The new biomes often have a unique gimmick.
As for how much playtime that will net you depends entirely on how fun you find the new biomes and how often you visit them (they form optional alternate routes). Getting all items and achievements can go as low as 3-5 hours per DLC if you blaze through everything on 0 BSC. If you play on higher difficulties, want to unlock all the outfit blueprints, etc you can get dozens of hours out of the DLCs.
If you're lukewarm on the main game, the DLCs will likely not change your impression. If you like the main game and especially if you start getting bored by visiting the same biomes fighting the same enemies then the DLCs will add variety and challenge (on average DLC biomes are a bit more difficult than main game biomes).

[Brakkolan]

which one is the easiest? I'm bad at the game, I didn't even beat the final boss once but after 20 hours I want more variety, which one should I get first?

[Gatebase]

The Bad Seed gives an alternate route from start to first boss.
Fatal Falls gives an alternate route from first boss to second.
The Queen and the Sea gives an alternate route from second boss to a new ending (with a different final boss).

So by virtue of their placement in the run, early biomes are going to be easier than later biomes. But also design-wise I think this holds true - the DLCs only got harder with every new one. Beware, the latest DLC is also by definition to least practiced, so it might just "feel" harder.

TBS has two fairly traditional levels and a decently hard boss (certainly harder than the Concierge, probably not harder than Conjunctivius? hard to say). FF has more unconventional level design (one I hate and think is really hard and one I absolutely love) and the boss is fairly in line with Timekeeper or Giant. TQatS has a regular level (certainly not absurdly hard like the Distillery) but then goes into two back-to-back bosses that are probably about as deadly as Hand of the King if not more.

So I definitely don't recommend the latest DLC for you, but among the earlier two it's up to you which part of the base game you want the variety on. TBS is easier to explore because it comes early in the run and isn't as punishing if you die. Personally I don't like the swamp style of TBS and think FF is more interesting, but that's just my personal choice. Also note that on TBS and FF the first level allows you to go back to regular levels afterwards, but going to the second level locks you in for the new boss of that DLC.

[The destoyer of worlsd!!!]

Fatal Falls probably adds the single most compelling alternate biome of them all, since in the Fractured Shrines, not only is a legendary item guaranteed to be available there, you can even choose which one of three you want

I'm not digging Undying Shores as much, though, because unlike Fractured Shrines where you can just go to the Clock Tower immediately afterwards, the only way forward from there is to beat a boss that's far harder than it has any right to be