Field Sets are usefull only when Visualforce (and/or Apex) is involved. You can't use them to "merge" a set of fields in a standard page layout.
You can go with a single text field for the address, as long as you don't want to use specifically one of the address field (such as "Country") in your report, validation rule
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https://success.salesforce.com/ideaView?id=08730000000BrVWAA0
Field Sets are usefull only when Visualforce (and/or Apex) is involved. You can't use them to "merge" a set of fields in a standard page layout.
You can go with a single text field for the address, as long as you don't want to use specifically one of the address field (such as "Country") in your report, validation rule