Also, what're the differences between them?

Gelbooru has everything Danbooru has and more. We literally import everything uploaded to Danbooru, so when you factor in what our users upload, we have more than they do, hands down. Sankaku, on the other hand, has twice as many images as we do, though I get a feeling that number's padded a bit with duplicates.

Danbooru is probably the most advanced with features, and has the most users actively tagging its posts, which makes things nice and easy to find. The downside to Danbooru is generally that they're very picky about what content they allow, and they throttle your access to stuff pretty heavily if you're a regular user by limiting how many tags you can search at once and even blocking certain tags if you aren't a premium member (though that one isn't really their fault - if they didn't do this, they wouldn't be able to do J-List ads).

Gelbooru isn't quite as advanced (yet - we're working on that!), but we have more images. Not just because of the reasons mentioned above, but also because we're a lot less picky about what content we allow. We also give access to the entire site, everything we have available, to users who haven't even registered an account - you can search for as many tags as will fit in your HTTP request packet (something like 4kb of text if you don't have anything in your blacklist), and we do not have any tags behind a paywall.

I honestly have no idea how advanced Sankaku is, because I can't stand to use their site.

Sanakau has everything Danbooru does because like here, they get imported by their admin/bot account.
chan.sankakucomplex.com/user/show/1 . They also have almost everything Gebooru has because a lot of Gelbooru uploads are now being uploaded by their gelboorubot chan.sankakucomplex.com/user/show/322790 . Originally their admin/bot handled anything uploaded from here. They also have several other active bots that grab uploads from other imageboads. The site has way more content than here or Danbooru, but the flipside of that is in general, they have ha a lot of lower quality content because they allow western art and their bots and uploaders tend to upload duplicates. But at the same time, a lot of Japanese content that isn't here or on Danbooru yet. And like here, you don't pay to see certain tags, though you do need a account to view anything tagged blocked_artist.

Stuff like toddlercon and extreme guro are allowed, but IRL uploads aren't, they must be uploaded on Idol Complex, no exceptions. If you're a regular member, your uploads go through mod que, and may go unapproved for years depending on the quality, if you bothered to tag your upload, and if it's not a dupe. Or is anyone who comes across it like it. You also have a upload limit, if one is approved, your upload limit is increased by 2. Though regular members have a chance to get unlimited uploads, but they will still go through mod que. Contributors and higher automatically have their upload limit removed, and their uploads no longer go through mod que. And depending on the Contributor or higher, they can moderate (approve posts). Though most automatically gain that ability when promoted.

Since I can find stuff on Sankaku that isn't here, I would say Sankaku has the most content.

Sankaku is also much less picky than Gelbooru on the "has to be Japanese-made/related art" side of things (although there seems to be a huge drop on strictness on Gelbooru).

Source: I use Sankaku.

Not taking dupes or deletes into account, Sankaku does have more uploads overall. By about 2-3 million.
Danbooru and Sankaku also have flash support however. Though I doubt those alone make the difference. Occasionally I see images on Sankaku that are of decent quality and by Japanese artists, so I imported a few of them.