Observation- i may end up viewing based on my perspection only without getting the right result.

Questionnaire- they are always busy to answer unless through structured questions in the paper.

Interview- hmmm bankers manage to attend to their customers so to start attending to external bodies with questions it will take time.
So which one do you think is ok by you？

I think it all depends on the nature of your research topic , your research objectives and the variables you are trying to measure. If u had posted the topic, it will be easier to suggest a suitable research instrument.