What is the difference between "¿Qué pasa?", "¿Qué tal?" and "¿Qué onda?" ?
They all mean and can be translated toas "What's up?" in English per my understanding and they are all informal. Are some of them more slang? Are some of them more common in certain Spanish speaking countries/regions? Which one is more common in Spain?
Origins and literal meanings can provide clues but of course, they are idiomatic expressions:
¿Qué onda? literally means "What wave?" and wave can refer to vibrational waves, thus vibes. It is close to "What's the vibe?" (which I don't hear in English but there is some usage online)
¿Qué tal? literally means "What such?" but could mean "To What extent?" as well. (possibly)
More details in this question:
What is the origin of the expression "qué tal"?
¿Qué pasa? literally means "What passes?" or "What happens?". The verb pasar means "to pass" or "to happen" in this context.