The 'three ages' described here follow the terminology used in the Silmarillion. Perhaps slightly confusingly, the word 'ages' here describes long periods within the greater First Age.
Though Silmarillion is rather vague on the timescales involved, The Annals of Aman in volume X of The History of Middle-earth gives us exact dating for the Captivity of Melkor. Melkor was taken back to Valinor as a captive in Valian Year 1090, and bound with the chain Angainor in VY 1100. After this, he remained a captive in Mandos until his release in VY 1400. Converting these dates to years of the Sun, the period from his capture to his release spans a period of 2,971 years.
Melkor was free in Valinor for some considerable time before the Darkening of Valinor. At first he laid his secret plans while feigning reform, and later fled from Valinor after those plans were revealed. In total it would be no less than 890 years (of the Sun) before Melkor struck down the Two Trees and escaped into Middle-earth.
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