For "the web"? HTML, CSS, and Javascript. If HTML doesn't count as a real programming language, add SQL. You might be looking for an answer along the lines of a

For "the web"? HTML, CSS, and Javascript. If HTML doesn't count as a real programming language, add SQL. You might be looking for an answer along the lines of a backend language like PHP, ASP, or JSP, but they all fill the same need of producing HTML code based on whatever data you have in the system. If I could only know three languages in general, I would choose Python for a high-level structured language, C for a low-level structured language, and Lisp for a different way of thinking about problems.