This doesn't mention having the shooter in his sights for 3 minutes, but it does say that the snipers saw the shooter on the roof 20 minutes before any gunfire.
Officials said the snipers spotted the suspect, 20-year-old Thomas Matthew Crooks, on the roof of a building outside the security zone at the rally in Butler, Pennsylvania, at 5:52 p.m. ET. The shooting happened at 6:12 p.m. ET, 20 minutes later, the sources said.
...
According to the sources, the timeline presented in the briefing was as follows:
- 5:10 p.m. Crooks was first identified as a person of interest
- 5:30 p.m. Crooks was spotted with a rangefinder
- 5:52 p.m. Crooks was spotted on the roof by Secret Service
- 6:02 p.m. Trump takes the stage
- 6:12 p.m. Crooks fires first shots
Twenty minutes passed between the time USSS snipers first spotted Crooks on the roof and the time shots were fired at the former president, new information shows.
abcnews.go.com
ETA: The article does mention "3 minutes" here...
New analysis by ABC News' visual verification team reveals that one of two sniper teams posted atop buildings to the north and south behind the rally stage repositioned before the first gunshots were fired. At 6:09 p.m., three minutes before the first gunshots rang out, the sniper team on the roof of the building south of the stage, the furthest away from the shooter’s location, repositioned, turning from the south to the north in the direction of the shooter’s location.