Yes, if you choose Chinese, you can I still study something unrelated to Chinese in university. And yes, you only need 3 A levels (in the vast majority of cases).
Some courses at university don't require that you have studied anything in particular at A level. Others have a particular subject or group of subjects in mind, but don't care about the rest.
For example, if you want to study Computer Science at uni they'll almost certainly want you to have done Maths at A level, but don't mind what else you've studied. For other degree courses, they might mandate "two maths or science subjects", or something like that.
Do you have any idea what you might like to study at uni? Other than Chinese, what A levels will you be doing?