We use both apartment and Flat here @ India! BTW, whats a Condo? Are you experimenting something with these words?
Condo is an apartment/flat basically. " Condominimum" . In USA no one is using the word " flat" for some reason. I wonder if it's the only English-speaking country that does not use that word or there are others. I want to make a map of sorts so yes.
We use the term flat or unit sometimes apartment here in australia although the term condo has been mentioned in some area's but most stick with either a flat or unit.
Flat in the UK. But some people use apartment. It's the American influence. Condo is mainly used in Canada and USA, but it's more of a legal term than to describe physical property (it's an alternative to leasehold, i.e. you are freeholders but still have some shared responsibilities for common areas).
Condo is normally regarded as higher end, while apartment is lower end. We don't use the term flat around these parts (Texas).