Than is used after adjectives and adverbs in the comparative degree. As and that are not used after comparatives.

• John is taller than his brother. (NOT John is taller as his brother.) (NOT John is taller that his brother.)
• She earns more than I do. (NOT She earns more as/that I do.)

As…as…

The structure as…as… is used to say that people or things are equal in some way. Than and that are not used in this way.

She is as tall as he is. (NOT She is as tall than he is .) (NOT She is as tall that he is.)

As and than clauses with missing subject or object

As and than can introduce clauses in which the subject or object is missing. Note that these structures are mainly used in a formal style.

The meeting was as successful as had been expected. (= The meeting was as successful as it had been expected.)
They gave me more than I had asked for. (NOT They gave me more than I had asked for it.)
Don’t forget to take your ticket, as I did yesterday. (NOT Don’t forget to take your ticket, as I did it yesterday.)