IT companies

It depends on the company, and on what you want to do for the company. The three languages you mention are all still in use. Most companies also have need for JavaScript expertise. Python and node.js are pretty popular. Ruby was hot but seems (to me at least) to be in decline (but is still being used).

If you want to pick the most useful (i.e. used at the most companies), I would pick up HTML5, JavaScript and make myself a web developer (since the the majority of companies need web developers).

Its all about the today's requirement in the market select that.