To 'consider' and 'think over' are the same - it means you are thinking about something that requires you to come to a decision, for example, you have to 'consider' or 'think over' wether to have a red car or a blue one. To 'think about' means that you can simply be thinking about anything - with or without having to make a final decision, for example, you can 'think about' owning a red car one day maybe.