“Is Abidhamma different from commentaries? ”
Yes. Abhidhamma is different from the Commentaries.
– Tipitaka means “three baskets”: Sutta Pitaka, Vinaya Pitaka, and Abhidhamma Pitaka.
Yes. Patisambhidamagga, Petakopadesa, and Nettippakarana are the 3 original commentaries.
“What about Milindapanha , since it is in the Khuddaka Nikaya too?”
– Milindapanha is not a Commentary. It has recorded conversations between an Arahant (Nagesena) and a Greek King (Milinda) who controlled a part of India at that time (a descendant of Alexander the Great). He was a Buddhist and asked a good set of questions.
More historical information at:
“Historical Background“