Yes you can! Hill climbing ability depends on gear choice and rider conditioning. Also, the foot platforms on all StreetStriders are designed with a rear tilt that lets you push backward on the foot platforms while you lean forward to climb hills, giving you more hill climbing power.
The 3i has 3 gears and the 7i has 7 gears. The ratios of gears 1, 2 and 3 of the 3i are the same as the ratios of 3, 5, and 7 of the 7i. With the 3i, moderate hils are climbed. Gears 1 and 2 of the 7 speed are for climbing even steeper hills.