An RF3 is not really a filter . . . as in tuned to a specific spot frequency or a band of frequencies. It is a pair of chokes, wound on a common former, which act by blocking any
common mode signal whilst allowing the
differential mode signal to pass.
Remember that an xDSL circuit operates in differential mode. So to minimise the effect of common mode RFI, the two legs of the circuit should be balanced. The better the balance, the less the effect of RFI upon it.
JGO has provided a good
description of an
RF3's mode of operation.