use Excel To Table (Conversion)—ArcMap | Documentation (arcgis.com)
and see if it comes in properly as a text field.
If that doesn't work, you may need to do a field calculation in a text field using something similar to
(replace "a" with he field
"{}-{}-{}".format(!a![:3], !a![3:6], !a![6:])
... sort of retired...