Positive degree: the base form of the adjective or adverb. It does not show comparison. Comparative degree: the form an adjective or adverb takes to compare two things. Superlative degree: the form an adjective or adverb takes to compare three or more things. Use the comparative degree (-er or more form) to compare two things.
Your memory is better than mine.
Donald Trump is more successful than Donald Duck and Don Ho.
Use the superlative form (-est or most) to compare three or more things.
This is the largest room in the house.
This is the most awful meeting.
Never use -er and more or -est and most together. One or the other will do the trick nicely.