You would either use a IF() formula or create a table for Office Locations and use a Lookup field.
The formula option would be something like this:
IF(FIND('Office A', {multiple select field}), 'the address for Office A')&IF(FIND('Office B', {multiple select field}), 'the address for Office B')&IF(FIND('Office C', {multiple select field}), 'the address for Office C')
The new table option would have one column which names each building and another column for the address. Your original table would use a Link to Another Record field to connect to the new offices table instead of a multiple select field, and your address column would be a Lookup which will reflect the addresses from the new table.
Either option should work for you.
