If we know the three specific digit values, the probability that an 11-digit phone number will contain only these three digit values is:
P = (3/10)^11, approximately 0.00000177,
simply the product of the probability that each of eleven digits will be one of the three specified values.
Note that this does include combinations that utilize only one or two of the three digit values.
If the item of interest is that an 11-digit phone number will contain only three digit values or fewer, but these can be ANY three digit values, we need to multiply the probability corresponding to any specific 3 digit
values by the number of such
combinations of digit values. That does involve factorials:
P = (3/10)^11 * 10!/(7!3!) , approximately 0.000213