1 "想" sounds more informal than "想念" and "思念". In speech, I never say "想念" or "思念".
2 In contrast to "想" and "想念", "思念" means a greater degree to which you miss someone.
3 "想念" and "思念" can be followed by things as well as people, while "想" is rarely followed by things.
e.g.
You can say 我想念/思念祖国.
But "我想祖国" doesn't sound very natural.