Where I'm from, labourer (in Commonwealth spelling) generally only refers to unskilled or semi-skilled manual labourers. It is used a lot more to describe workers in the past in an academic setting, but I think that's largely because workforces of the past did tend to have a greater proportion of less skilled and more manually involved workers.
Worker can really refer to anyone who works at a job, even including members of middle management depending on the context. If you have more specific questions regarding the use of that term they might be more useful to ask.
Workman is pretty informal (in Australia, anyway) and I've heard it most commonly used when referring to tradesmen and/or labourers who are working in public, especially on construction projects in the street and roadworks. It helps that they're extremely male dominated work environments here and the term can be used (though informally) with little confusion.