What should our attitude be towards the first three Khalifahs?

All the companions of the Prophet Muhammad (PBUH) are to be respected other than those who perpetrated injustices.
Allah tells us in the Qur’an:
Those who confront and disrespect Allah and His messenger, Allah has cursed in this world and the hereafter and prepared for them a disgraceful chastisement. Those who disrespect believing men and believing women without reason bear the guilt of slander and manifest sin. Qur’an 33:57 – 58