I don't doubt a quad copter could hit 10k feet, but I do doubt many people would go that high. The only way to control that would be via a remote telemetry and/or video link or via strict autonomous programming. A standard 250 class quad could likely hit that 3km in 1.5-3 minutes under full power. I know from my time build and flying that a flight time for a quad hovered (hah) around 15-20 minutes on average. That leaves 9-12 minutes of loiter time as max. Then you need to factor in the power cost of stabilizing the aircraft at the height. The wind will be shoving it around so the FC will need to be constantly fighting for position hold.

I'd really love to see the math on the probability of a tiny quad intersecting the flight path of a firefighting aircraft at 3km altitude though. They are small and that's a LOT of open space. I can barely spot one against the open sky at a few hundred feet up. If these guys spotted one they must have amazing superhuman vision or amazing luck to have come within a very close distance.

Personally I put most of the sightings into the mass-hysteria camp. I know there are idiots out there flying in controlled space, but I also think things are being blown way out of proportion.

Edit: I found a site that did the speed/climd rate calculations. My numbers are bit off. The DJI, a common platform, could hit 3km in about 5.5 minutes and a 250 racing quad could do it in about 5. With about 20 minutes of battery power that leaves maybe 5-10 minutes of loiter time.

DJI Phantom 2 Vision+: flight speed of 17 m/s (61 km/h) and a maximum climbing rate of 9 m/s

250 FPV Racing Quad: maximum flight speed of 18.5 m/s (67 km/h) and a maximum climbing rate of 10 m/s

source: http://bpmsg.com/flight-speed-of-a-quadcopter/