calamity最重
adversity和misfortune程度一样,可互换
mishap最弱
字典为证（柯林斯字典）：
A calamity is an event that causes a great deal of damage, destruction, or personal distress. (FORMAL)
Adversity is a very difficult or unfavorable situation.
= misfortune
A mishap is an unfortunate but not very serious event that happens to someone.