Needn't mustn't and needn't have

Needn't - you don't have to do it = do / does not need (it not necessary to do it)

Mustn't - Don't do it. 

Needn't have done - it was necessary to do it but you did it. 

For example:

I needn't buy an Ipad because I already have Samsung galaxy. 

I don't need clean my apartment because it isn't dirty.

I needn't have walked home. I could have caught a taxi.