Im going to second the notion of swapping your gears or going with a smaller ratio, right now it is at 3:1 which means every 1 rotation of the motor output shaft you get 3 rotations of the wheel. When you first see this you think, “Sweet I want to go fast that sounds great” the catch is you will not be able to accelerate quickly. This stems from the fact that when you increase speed you decrease torque. Force = torque /(radiussin(theta)) Using that and F=MA we can calculate your max acceleration to be A = motor_torque/(wheel_radiusrobot_masssin(theta)) For the sake of simplifying we can assume that sin(theta) = 1. Using that our equation goes to A = motor_torque/(wheel_radiusrobot_mass)
Here is the my issue with the way you have this system geared, you may have a top speed of 3motor_speed but you will more than likely never see that unless you run down a long hallway. Recall v = v_0+at, assuming v_0 is 0 (starting from stand still) you could calculate how long it would take you to reach any speed.
As for the turning issue, I’ll let someone else show you how to work out the math for that.