Sus and add chords

You are definitely right with altering the 3.. The cromatic order is sus2, minor3, major3, sus4. I don’t know about the madd9 and m9.
But I recommend watching lesson 48 and 49. They are real eye-openers to sus2, sus4, add9, maj7, 7,b5 , aug, etc.
Before these lessons I’ve been confused about maj7 and 7 and I was really excited after watching them.
The lessons bring the chordbuilding to the point. Probably you have known some things about it before but in my case it was never that clear in my mind.

The main difference between suspended and added chords is in the triad. When using suspended chords, the triad is altered. For sus2 chords, we have to play the 1, 2 and 5 notes (C-D-G for a Csus2 chord). Same thing for sus4, the difference being that we play the 1, 4 and 5 notes (C-F-G for a sus4 chord).
Added chords however, as their name suggests, are all about adding a note on top of a triad. For example, add9 chords have the following formula: 1-3-5-9. In other words, we keep the triad, but we also add the 9th note (which is the same as the second note from the major scale, but an octave higher) on top of it. Cadd9, as a result, would comprise of the C, E, G and D notes. This should be it in a nutshell. As Jen said, they are explained in some lessons (I don’t remember which ones, so I’m going to take Jen’s word for it hat the lessons are 48 and 49).