3.5 million is possible and realistic, here's why:
Planetary Annihilation's trend 5 days before it finished was about 1.9 million.
Using stretch goals, Uber was able to up the trend to 2.2 million in 5 days.
Project Eternity has 5 days left and its trend is currently at 3.1 million.
If we assume the same relative growth for PE, we can expect PE's final trend to end up at (2.2/1.9)*3.1 = 3.6 million!
Fingers crossed