Hello keywords are important for every website why because user search queries. Long tail and short tail keyword both are important but long tail keyword takes short period of time to get top position on major search engine than single word keyword. Hope you understand!
If you are choosing a single word as keyword, give 2 to 3 keywords which is related to the main keyword , If you are choosing a key pharse of 3 to 4 words , then one keyword phrase is enough.
Check competition first for 2 phrase and 3 Phrase if your site is new then its good to start with long phrase once start performing in Search engine u may go for narrow keywords.
2 phase or 3 phase is not more important but important is which phase is best suit for your website....