You will not need a visa for hong kong, if you are american. However, if you have a china visa and enter Mainland China firstand go to HK Macau or taiwan. . You will need a new Visa to return to China. If your visa is not a dual entry or multi entry, you can only use it for one entry into mainland China. You can ge all of these facts by visiting the website of the China Consulars office in your country. Remeber, entry rules and visa requirements change often so it is best to view the embassy and consular websites.
Of courst NO. China visa holders can enter Mainland China only. You can't use it to visit HK, Macau or Taiwan. Actually, many nationalities can enjoy visa-free to visit HK not more than 90 days. (except some small and poor countries). Besides, Taiwan issues its own visa because Taiwan government is not managed by China Central government. If you are an US passport holder, you can visa-free to stay in there for 30 days. It is very convenient to visit HK & TW. You only need to apilicate the China visa. Have good days there!
NO. Depending on where you are from you will need separate visas. If you are from UK, Canada, or US, you do not need a visa prior to entry into HK, they will give you a 180 or 90-day stamp (UK is 180 days). As for Taiwan, it is considered to be part of China as a "breakaway" province. Either way you need a separate visa.