딱 means "Just" when you use it you only point to thing specifically, example: 넌 딱이야 = Just you "the one"(pointing to the person only him /her and nobody else )
또 means "again /too" ,example: 또 할거예요 = I will do it again.
때 means "when (something happened )" ,example: 어렸을 때.... = when I was young /kid.... ,, another example: 봤을때....= when I saw...