While not a 3900, should be same/similar distance. My '14 3150RL has the springs on top of the axle and from the axle tube bottom to the ground is 13" The distance from the frame to the ground is 25".
I am parked on concrete, 6 point jacks down, have Bridgestone Duravis 250 tires.
2012 Ram Laramie 3500 DRW 4x4 3.73 Tow Max Pkg B&W Companion 60 gal RDS aux fuel tank. 2014 Montana 3150RL, 2 A/C's, Leather, 6 Point Jacks, Splendede WD2100XC, Mor/ryde X-Factor, Duravis 250 tires with TST 507RV monitors. 2 x Honda EU2000's