1
She was thinking in you all the time while you were abroad.
She was thinking of you all the time while you were abroad.
She was thinking about you all the time while you were abroad.
Use think of or think about someone, not think in someone.
2
I am thinking to go to the south for a few days.
I am thinking of going to the south for a few days.
I am thinking about going to the south for a few days.
When you are considering a plan, use think of/about + -ing: ‘I’m thinking of buying a new phone.’
3
I think I don’t have a favourite colour.
I don’t think I have a favourite colour.
To say that you are unsure something is true, we normally put the negative with think: ‘I don’t think I have a favourite colour.’ The purple sentence is possible, but this is the more natural way to express the idea.