Need to add counties to my spreadsheet that has city state and zip

address city state Zip county
510 Spann Street Mount Pleasant SC 29464
103 W 8th St Chickamauga GA 30707
4707 Murray Lake Ln Chattanooga TN 37416
816 kay cr chattanooga TN 37421
343 Durham St Soddy Daisy TN 37379
1541 Burns ave CHATTANOOGA TN 37412
480robindrive Chattanooga TN 37405
4719 Murray Hills Dr Chattanooga TN 37416
50 w.m.l.king blvd. apt.404 CHATTANOOGA TN 37402
122 TUSCANY CT Ladson SC 29456
1911 calvert st. apt, c North Charleston SC 29405
2605Elms plantaion blvd #713 CHARLESTON SC 29406
1217a Arkansas dr Ladson SC 29456 Ladson SC 29456

Thank you for the info; so to summarize...

1) Download a "us zip code list with county
2) use this list to add counties to my spreadsheet;

I have over 2,000 individual addresses that need to have the county added to them, so what would be the quickest method
to do this? If is is a formula, I did post a small sample of my csv file I am working with, can someone explain how I would
add a formula to the sample I provided so I can translate that to the actual large file I have.

Thanks in advance to anyone with the patience to help a newbie; or maybe suggest a tutorial video on you tube? or course?