Thames Valley Police is appealing for witnesses to an attempted robbery in Furzton Lake, last week.
Just after midnight on Tuesday, the victim was running around the lake when three men asked for the time and to borrow his phone.
He let them and then they pulled out a knife and threatened to stab him if he did not give them the phone.
The victim then fled the scene uninjured.
All three offenders are around 20- 22 years old, slim, 5ft 10ins and wearing dark clothing.
Anyone with information is asked to contact Det Con Wendy Cissell via the Thames Valley Police Enquiry Centre on 101.