It’s still doable. The average GPA for the fall 2015 class was 3.72 (unweighted) and Mechanical Engineering is one of the toughest majors to get in as they only took 24% of those who applied in 2015. SATs (reading, math) and GPA are all they look at. However, if you reach 3.39 and score well (1250-1300 in the SAT for reading and writing) you may be competitive.
https://en.wikipedia.org/wiki/Cal_Poly_Pomona_College_of_Engineering