I can come up with several hypothesis as to why that can happen.
First, the frequency of the phones for transception is 900MHz, or maybe its up to 1.5GHz or so...that's near microwave frequencies (off the top of my head without looking at the frequency spectrum), which would be the right frequency to pop popcorn.
The second is that the phones put out milliwatts of power in order to make the transception capable of reaching a nearby antenna, and despite the FCC regulations, may be enough to pop popcorn.
The third theory is that if the three phones combined put out enough wattage and with different frequencies can come up with different harmonic frequencies, it would be enough to microwave-pop the popcorn. I would suggest not having more than one phone too close to your brain, much more so, don't talk on the phone too much at one time. \("_")/