The offer extends to all listed Macs and iPads. For example, customers can today order a Space Grey 2017 13-inch Apple MacBook Pro with Intel Core i5, 8GBGB of RAM, 256GB SSD, and Intel Iris Plus Graphics 640 for £1,449 RRP, which includes the three-year guarantee at no extra cost, with an option for three years' Added Care (accidental damage cover) for £80.
Similarly, the retailer is offering a free three-year guarantee on a new Silver 27-inch iMac with Retina 5K Display, Intel Core i5, 8GB of RAM, a 1TB Fusion Drive and Radeon Pro 570 GPU for £1,749 RRP, with three years' of accidental damage cover for £30. The site lists the new iMacs "Coming Soon", but this is likely just an oversight, since the Add to Basket option has been added and several listings claim more than 10 in stock.
Also included in the three-year guarantee offer are Apple's new range of iPad Pros, which the online store currently lists as out of stock as the retailer waits for them to become available to resellers later this month. Given that the offer is open until June 28, John Lewis is presumably confident of receiving the iPad Pros before then.
The retailer is also offering £50 off all iPhones when bought with an Apple Watch. A two-year guarantee is also included for both devices.
Note that configuration options are not available from John Lewis and all Macs are only available with the listed specifications. Equivalent three-year AppleCare from Apple costs £159. For more information on John Lewis inclusive guarantees, see here.