CC Ādi 5.16

tāhāra upari-bhāge ‘kṛṣṇa-loka’-khyāti
dvārakā-mathurā-gokula — tri-vidhatve sthiti
Word for word: 
tāhāra — of all of them; upari-bhāge — on the top; kṛṣṇa-loka-khyāti — the planet known as Kṛṣṇaloka; dvārakā-mathurā-gokula — the three places known as Dvārakā, Mathurā and Vṛndāvana; tri-vidhatve — in three departments; sthiti — situated.
Translation: 
In the highest region of that spiritual sky is the spiritual planet called Kṛṣṇaloka. It has three divisions — Dvārakā, Mathurā and Gokula.