They have all the same meaning, but underground is more general and official. Tube is a colloquial name for underground in London and metro often means underground in other countries. Also, for example, in Manchester Metro is name for trams. Another name for it is subway, but this is american English. Hope it helps.

In most American cities we say "subway". "Metro" is used in Washington, DC, but people will understand if you say "subway". "Metro" may be used in other American cities as well, but my subway experience is mostly limited to the mid-Atlantic and northeastern USA. In Boston it's called the "T", short for MBTA, or Massachusetts Bay Transportation Authority. In some cities the subway is known by an acronym. In SanFrancisco it's called BART, for Bay Area Rapid Transit.

I think it depends on location. Most cities in the US don't have underground subways, so a metro usually means a bus or train(or even a trolley), or even both. Metro is just short for Metropolitan. I'm guessing someplace you have been uses tube as slang for an underground train?

对于一个伦敦人来说：想说’地铁‘那个词的时候，我们百分之九十九时说‘tube’。我们伦敦地铁公司称为'London Underground'，所以‘underground'是比较正式的说法。巴黎的地铁是'metro'。伦敦人从来不说’subway‘、’metro‘来描述自己的地铁，反而英式英语里的'subway'是'行人隧道'，还有’metro'是'metropolitan'的缩写。美国的说法跟我们不一样。

metro in America equals underground in England tube different it means a hollow circle like a tube of toothpaste