[想 + v] means [want to + v]

[要 + v] can mean either [want to + v], [demand to] or [need to + v] 

[想要 + v] means [want to + v] or [attempt to + v]

- When you want to say "**need to**" you can only choose "**要**" among the three

- When you want to say "**want to**", you can use "要", "想要" or "想"; Using "**想**" makes it unambiguous because it doesn't contain other meanings

- When you want to say "**attempt to**" you can only pick "**想要**" among the three 

Note: With a clear context, you can deduct 要 means "want to" or "need to" in most cases

Examples:

我要去美國 = I want to/need to go to America

我要去美國度假 = I want/demand to go to America for a vacation (a vacation is something you want to do in most cases)

我要去美國公幹 = I need to go to America for a business trip (going on a business trip is something you need to do in most cases)