Limit and Offset

Here we have a transactions table where we store data on purchases.

CREATE TABLE transactions (
   transaction_id int unique NOT NULL auto_increment,
   product_id int NOT NULL,
   sold_at datetime NOT NULL,
   price float(10,2),
   customer_name varchar(20) COLLATE utf8_unicode_ci NOT NULL,
   PRIMARY KEY (transaction_id)
) ;

Please, select 3 unique customers that made transactions in our store.