Yes, these three terms are basically the same thing.
Non-diatonic refers to any pitch that does not fit within the normal pitches of a standard scale.
There are several different kinds of scales so it depends on the context whether the pitches are non-diatonic. For example, the non-diatonic pitches of a natural minor scale will be slightly different than a major scale or even a harmonic minor scale.
Chromatic comes from the Greek word chromos which means color and is another word for non-diatonic pitches.
In a narrower context it is typically used to refer to passages of consecutive notes where the pitches are all a half step apart.
Accidental is the overarching word for sharps, flats, and natural signs in music that are not within the original key signature. These are generally non-diatonic pitches.
Whether accidentals are diatonic to the scale or not can get a little blurry with minor scales as the key signature usually reflects the natural minor while melodic minor is prefer for composing melodies and harmonic for the harmonies--all of which have slightly different pitches. For example, an accidental may be diatonic to the harmonic minor scale being used but not a part of the written key signature.