Ask HN:What should i learn after html/css?

Well a web developer told me to learn html css javascript, but is there a specific reason there 3 are used in web. dev. ? i also notice google code university recommends those 3.

what other languages could i learn in under 3(edit:basics in 3-5 weeks) weeks, that will be useful?

edit: later with this background, i want to go into iPhone / Android development.