What is the difference between a marquis, a viscount and an earl? What is their rank order?

Marquis gt; earl gt; viscount.

European countries have high and low titles, but there is no obvious subordinate relationship.

Generally, a duke can control the earls in his own territory, but the amount of control depends on the number of earls in his territory that the king entrusts. There is no formal system. The earl controls the viscount and the baron as well.

There is no fixed number of noble servants, and it is generally determined based on their property status (mainly territory).

As long as they are earls, both men and women are high-ranking nobles. Generally speaking, their marriages are based on family connections (that is, marrying to nobles). But there were also marriages to lower-level nobles or commoners.

John of Gaunt inherited the dukedom brought by his wife.