Neel range on which the country people stay.
Suvel range which is the vast plateau.
Sundar range where Ashok Vatika is located and the total events of sunderkand took place on this range, hence the sunderkand.
Further more Shri Tulsidasji has said;
“Sundere sundero ram sundere sunderi katha
Sundere sunderi sita sundere Kim na sunderem”
Meaning of this is, in sunderkand everything is sunder that is beautiful. Rama is beautiful; Sita is beautiful the story is beautiful. It is an upasana or a prayer of beautiful people hence the prayer has to generate a power for the devotee or sadhak or the reciter. It contains a lovely hugging of lord Rama and his devotee Shree Hanumanji showering the word of appreciation and indebt ness for all good Hanumanji did to Rama while rescuing Sita. A devotee to whom Rama could not reciprocate. He did as a sevak. This power percolates to the devotee who prays to lord Hanumanji via sunderkand containing the praise of lord Shree Hanumanji.