There is some overlap, but 'customers' is mostly used when people pay for a product or service from a commercial premises or organization, like a shop, or phone / energy provider. 'Clients' usually pay for someone to do something for them, or on their behalf, such as with lawyers or other professionals. 'Guests' is mostly used for people who stay in hotels, but is also used when someone ( a 'host') invites someone else, either for a meal, or to stay in their home. (offered for no charge).