travelling heathrow 3 to kings cross

Piccadilly line tube all the way from terminal to KingCross in just over an hour. The tube line starts at Heathrow so you will be guaranteed a seat and space for luggage. Cost £5.60pp. No need for a ticket, just use a contactless card or phone pay to tap in at either end.

You could also take the Elizabeth line to Farringdon and change to a Thameslink train to King’s Cross. The trains are bigger and so slightly more comfortable but you will need to change so a bit more hassle and it won’t be any quicker, but will be £13.30pp.

IMO It is the tube, Piccadilly line to King's Cross.

There are other ways, but they involve changing transport. They don't save enough time to make the hassle worth it to me.

A third vote for the Piccadilly Line. Just remember to check if there will be scheduled works the day you plan to travel. That happened to us last June prior to the start of our RS Best of London Tour. We were able to take the Lizzie Line to Bond Street and then walk to our hotel in Mayfair, though.