Most teachers regardless of whether they teach piano, guitar, voice, or tuba, should be teaching you the basics of music theory. This should include key signatures, time signatures, note durations, and how chords are made, inverted and arpeggiated. Since you mention major, minor, diminished I assume you are learning western music vs other world music. Because by in large many accordions have buttons that play chords perhaps your teacher is not so concerned about chord construction?
Traditionally in western music chords are built on scales. There are many types of chords but the most basic chord is the diad, or double stop, two notes that are sounded together.
Typically, diads are major or minor 3rds or 6ths, or 10ths, and often compliment each other.
However diads can be ambiguous as far as defining tonality. Since they are composed of only two notes it can be difficult at times to determine the exact scale one is playing unless there are enough of these to define a scale.
On the other hand, triads are three note chords that generally leave no doubt about their tonality and scale being performed. A major triad is played with the root, maj. 3rd, and perfect 5th.
In a major scale you will have the following chords from tonic to tonic: major chord (tonic), minor chord (supertonic), minor chord (mediant), major chord (subdominant), major chord (dominant), minor chord (submediant) and diminished (leading tone). All of these chords have a scale degree as the root note, then a 3rd and then a 5th (not always perfect).
More here but referenced to guitar:
Chords can have more than 3 notes, there are dominant 7ths and 9th chords and even 11ths thrown in as you build your repertoire to include music from the classics to jazz. Also chords do not have to have all the members played, you can easily have a root, a 5th, a 9th in one chord.
Chords can be implied by arpeggios so the notes do not all have to sound together, but can imply the same by being played as a string of notes.
Also you will learn that in the 20th century many western music composers abandon tonality for atonality.
In these works you will find no sense of a key center, and you may find the use of tone clusters which are chords made of several notes in an unusual order. Or mega chords that might expand the entire range of the piano.
Things you can do without buying another text book. Google and search Youtube to hear and see how the following key words will help you learn about music theory:
...and get specific with adding 'accordion' to your search as well.