A Major triad consists of a major 3rd (root to middle note) and a minor 3rd (middle note to top note) that is stacked on top of the major 3rd. An example of a major chord would be F - A - C
Augmentation means to get bigger. Unlike a Major triad, an Augmented triad consists of 2 major 3rds stacked upon each other (major 3rd between the root and middle note, and major 3rd between the middle note and top note). To make a triad augmented from a major triad, raise the fifth or top note by a half step. For example, if you want to make the major triad of F - A - C into an augmented triad, the top note C would be raised a half step and the augmented triad would be F - A - C#.
A Minor triad consists of a minor 3rd (root to middle note) and a major 3rd that is stacked on top of the minor 3rd. To change a major triad to a minor triad, lower the 3rd or middle note by a half step. An example of a minor chord would be F - Ab - C where the middle note A in a major triad is lowered a half step to Ab.
Diminish is to make smaller. Unlike a minor triad, a Diminished triad consists of 2 minor 3rds stacked upon each other (minor 3rd between the root and middle note, and minor 3rd between the middle note and top note). To make a triad diminished from a minor triad, lower the fifth or top note. To make a triad diminished from a major triad, lower the third (middle note) and fifth (top note). For example, a diminished triad would be F - Ab - Cb where the third (middle note) and fifth (top note) are lowered by a half step compared to the major triad (F - A - C).