They've all got their place in English, examples would be 'I'm thinking about going to live in Spain', the usage here indicates a thought process about the possibilities involved in maybe moving to Spain. 'Thinking of you' is quite a common phrase in messages or cards sent to someone you haven't seen that day or in a little while, you could also say 'thinking about you'; there is a subtle difference here, 'thinking of you' suggests that you are regularly thinking about the other person, 'thinking about you' is a little more specific, i.e. implying something like 'I'm thinking about you, wondering how you are, if you're ok etc. etc.' You can also say 'thinking on it', but it is unusual, it's more of an idiomatic phrase, e.g. imagine you ask someone a complicated question and they go silent, you wait a while and ask them what they think - they could reply 'I'm thinking on it' (but would probably use 'about it'). 'Thinking on it' is kind of a wild west, cowboy style of speech, think of the character Tommy Lee Jones plays in 'No Country for Old Men' (if you've seen it); he's the kind of guy who might say 'I'm thinking on it'.