Explanation

The time marker 'for the past three years' describes an action that began in the past and is still continuing, which requires the present perfect continuous tense. 'Have been walking' is exactly that form. 'Was' is simple past and cannot combine with a 'for the past three years' duration; 'have' (present perfect) suits completed actions rather than ongoing ones; 'had been' is past perfect continuous, used only for an action completed before another past event. Hence (c) 'have been' is correct.