What were at least 3 things the council of trent determined should be changed?

I would list these three things:

Simony was condemned.Indulgences were no longer to be sold.Clergy were to become better educated.

Detail/context:

The Council of Trent, held over a span of years from 1545 to 1563,  served to reform some abuses that were acknowledged by the Catholic Church.  Mostly, though, the Council aimed to assert the full authority of Roman power and doctrine over the Protestant threat.  

Simony was the practice of buying and selling church offices.  The Council of Trent condemned such practices, which had been widely abused in the church and criticized by reformers.As for indulgences, the underlying principle of indulgences was upheld -- that the church had authority to grant reprieve to penance or time in purgatory. But the sale of indulgences was stopped.  The church recognized that the selling of indulgences had been an abuse and determined to end that practice.As for clergy education, seminaries were established and the Roman Catechism, also known as the Catechism of the Council of Trent, was commissioned by the Council and was published in 1566,.  The intention of these actions was thoroughly to improve the education of the church's clergy.