Srimad Bhagavatam Canto 03, Chapter 16, Text 22

SB 3.16.22

dharmasya te bhagavatas tri-yuga tribhih svaih
 padbhis caracaram idam dvija-devatartham
nunam bhrtam tad-abhighati rajas tamas ca
 sattvena no varadaya tanuva nirasya
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
O Lord, You are the personification of all religion. Therefore You manifest Yourself in three millenniums, and thus You protect this universe, which consists of animate and inanimate beings. By Your grace, which is of pure goodness and is the bestower of all blessings, kindly drive away the elements of rajas and tamas for the sake of the demigods and twice-born.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
The Lord is addressed in this verse as tri-yuga, or one who appears in three millenniums, namely the Satya, Dvapara and Treta yugas. He is not mentioned as appearing in the fourth millennium, or Kali-yuga. It is described in Vedic literature that in Kali-yuga He comes as channa-avatara, or an incarnation, but He does not appear as a manifest incarnation. In the other yugas, however, the Lord is a manifest incarnation, and therefore he is addressed as tri-yuga, or the Lord who appears in three yugas.
Sridhara Svami describes tri-yuga as follows: yuga means “couple,” and tri means “three.” The Lord is manifested as three couples by His six opulences, or three couples of opulences. In that way He can be addressed as tri-yuga. The Lord is the personality of religious principles. In three millenniums religious principles are protected by three kinds of spiritual culture, namely austerity, cleanliness and mercy. The Lord is called tri-yuga in that way also. In the Age of Kali these three requisites to spiritual culture are almost absent, but the Lord is so kind that in spite of Kali-yuga’s being devoid of these three spiritual qualities, He comes and protects the people of this age in His covered incarnation as Lord Caitanya. Lord Caitanya is called “covered” because although He is Krsna Himself, He presents Himself as a devotee of Krsna, not directly Krsna. The devotees pray to Lord Caitanya, therefore, to eliminate their stock of passion and ignorance, the most conspicuous assets of this yuga. In the Krsna consciousness movement one cleanses himself of the modes of passion and ignorance by chanting the holy name of the Lord — Hare Krsna, Hare Krsna — as introduced by Lord Caitanya.
The four Kumaras were cognizant of their situation in the modes of passion and ignorance because, although in Vaikuntha, they wanted to curse devotees of the Lord. Since they were conscious of their own weakness, they prayed to the Lord to remove their still-existing passion and ignorance. The three transcendental qualifications — cleanliness, austerity and mercy — are the qualifications of the twice-born and the demigods. Those who are not situated in the quality of goodness cannot accept these three principles of spiritual culture. For the Krsna consciousness movement, therefore, there are three sinful activities which are prohibited: illicit sex, intoxication, and eating anything other than prasada, food offered to Krsna. These three prohibitions are based on the principles of austerity, cleanliness and mercy. Devotees are merciful because they spare the poor animals, and they are clean because they are free of contamination from unwanted foodstuff and unwanted habits. Austerity is represented by restricted sex life. These principles, indicated by the prayers of the four Kumaras, should be followed by the devotees who are engaged in Krsna consciousness.
Srimad Bhagavatam Canto 03, Chapter 16, Text 21
Srimad Bhagavatam Canto 03, Chapter 16, Text 23