My husband and I used to insert our fingers in our anuses during our sex acts. I want to know if this is (1) Haram and Kabaer? (2) Haram and not Kabaer? (3) Just bad attitude?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Allah Almighty made it lawful for the spouses to get pleasure and satisfaction from each other's bodies with touching and watching, excluding the sexual intercourse during the menses or into (using) the anus.
The reason for that is the nuisance and harassment in the both situations, the blood in menses and lasting thick impurity inside the anus.
Islamic scholars mentioned that touching the source of impurity without necessity is forbidden. So that performance, which has been mentioned in the question is a disdained act by perfect human nature and its continuation could draw the person to more repulsive deeds.
What Allah Almighty made lawful to us is enough.
In view of the fact that there is no clear and fixed text which forbids this kind of act between the spouses, we cannot give an absolute ruling for this prohibition.
Allah knows best.