Since the rates of each device are independent of each other, and the user is authenticated when any one of the devices authenticates the user, we have the following information:

A user is rejected if and only if all three devices reject him (him is used throughout to represent him or her), therefore, the combined rejection rate becomes:

FR = fr1 * fr2 * fr3

So we know that introducing more biometrics systems lower the overall false rejection rate. 

Now, to calculate the rate that any one of these produces a false accept:

 FA = 1 - Niether 1, nor 2, nor 3 produce fa
 = 1 - [(1 - fa1)(1 - fa2)(1 - fa3)]
 = 1 - [(1 - fa1 - fa2 + fa1fa2)(1 - fa3)]
 = 1 - (1 - fa1 - fa2 + fa1fa2 - fa3 + fa1fa3 + fa2fa3 - fa1fa2fa3)
 = 1 - 1 + fa1 + fa2 + fa3 - fa1fa2 - fa2fa3 - fa3fa1 + fa1fa2fa3
 = fa1 + fa2 + fa3 - fa1fa2 - fa2fa3 - fa3fa1 + fa1fa2fa3