Measuring speed with 3axis accelerometer
An accelerometer measures acceleration, not speed. In order to get speed, you will have to integrate the acceleration data over time. Since this is in the discrete domain (you can only sample the acceleration a finite number of times per second) it is more of an approximation and can be related by the following formula where A = acceleration, V= Velocity, T = Sampling period.

V[k+1] = V[k] + T*A[k]

So every time the acceleration is sampled, you would need to evaluate this formula, then make V(k) = V(K+1). This is essentially integration.     

Also keep in mind that the acceleration data the sensor reads includes the force due to gravity, this is usually accounted for by subtracting 9.8m/s from the z axis readings, however in your case, since the accelerometer is spinning, the effect of gravity would likely (hopefully) cancel itself out by being applied equally on all axis.

Usually accelerometers need to be oriented precisely to avoid huge errors. For instance an angle error of just 1 degree would cause the velocity to be off by 1.7m/s after 10 seconds. For applications where the orientation is not fixed, a gyroscope is usually used in conjunction with the accelerometer to account for the actual orientation of each axis. Since the accelerometer will be spinning rapidly in your case, I cannot quite say for sure how it will behave. Since you only need relative velocities, it may work consistently enough for your needs, or it might not work at all, I can't say. Even with properly orientated accelerometers, calculating velocity produces significant errors since each small error in the acceleration is going to be added together over time. In my experience this error could be around 3 m/s after 10 seconds or more. Its worth a shot though, good luck!