You could try something like Data - Text To columns, by fixed length,
choose to seperate out the first three digits - then in another column
have the equation ="(" & A1 & ") " & B1

this assumes that you have the first three in the a column and the
remaining in b, you could also split off the last four at the same time
then have the equestion ="(" & A1 & ") " & B1 & "-" & C1 to have the
number format out to (###) ###-#### then just copy the formula down the
column