Snow Patrol, The Killers and Arctic Monkeys will headline this year's T in the Park festival.
Snow Patrol played the festival in 1995 before they were signed
The event in Balado, Kinross, will also begin a day earlier, with music laid on for the thousands of pop fans who spend the weekend camping.
About a third of the tickets for the 14 year old festival have already been sold - snapped up just days after the 2006 event ended.
The rest of the July event tickets will go on sale at the weekend.
A total of 174 bands will take to 11 stages, also including Scissor Sisters and Scottish artists, Paolo Nutini and the Fratellis.
Snow Patrol, who first appeared at T in the Park as an unsigned band in 1995, will return to headline the main stage on the closing night.
For the first time, there will also be performers on the stage from 1700 GMT on the Friday, including Lily Allen and The Coral - although only the 50,000 campers expected at the event will have access to the festival arena.
More tickets for the festival are to go on sale at the weekend
The extension still has to win the approval of the local authority, Perth and Kinross Council.
Geoff Ellis, of festival organisers DF Concerts, said: "We have always said that it's the incredible loyalty of the audience that makes T in the Park so special.
"To show our appreciation we wanted to give something back to them, and create an even better experience than the last, so it is with great pleasure that this year we're announcing a third day."
T in the Park bosses urged music fans to buy tickets from official sources to avoid falling foul of online scams or vastly inflated prices.