Lassie, known officially as Lowes Female 15 or LF 15, hatched three chicks at the Dunkeld reserve last summer.
She returned there on 18 March, two weeks earlier than expected.
Lassie took over from long-term resident "Lady", who reared 50 chicks at the site.
She was joined by her mate a week after her arrival and their nest was eagerly watched for the first egg by viewers around the world via webcam.
Lassie laid her first egg of the season last week.
Charlotte Fleming, Perthshire ranger for the Scottish Wildlife Trust, said: "This is an important milestone in the osprey breeding season, but there is still a long way to go before we can relax.
"The ospreys need to sit tight and carefully incubate their clutch for at least 30 days in all weathers, and also protect their eggs against any predators that might dare to intrude.
"Just this morning our female had to chase off a pair of crows that ventured too close to the nest.
"It's possible for ospreys to lay four eggs, as the previous female at Loch of the Lowes did so on two occasions.
"We will be keeping a close eye on her behaviour over the next few days just in case she has another surprise for us, but all of our staff and volunteers are more than happy with a clutch of three eggs for now."
Ospreys typically lay between two and three eggs in a season.
Once extinct in the UK, there are now around 240 breeding pairs of ospreys thanks to the efforts of nature conservation charities including the Scottish Wildlife Trust,
The Osprey Protection Programme is supported by the People's Postcode Lottery.
A young female osprey known as "Lassie" has laid her third, and likely final, egg of the season at the Loch of the Lowes nature reserve.