One of the things seen on footballers' jerseys is their sponsor's logo or tag. Chelsea has been sponsored by numerous companies such as the Fly Emirates, Samsung mobile, and even Yokohoma tyres.
The 3 digits or sign is seen on the Chelsea football club jersey represents 3.com. A renowned company from Japan is now the official sponsor of the club in the English Premier League.
In addition, Three, which has gone from being the UK's first 3G-only network in 2003 to possessing more 5G spectrum than the rest of the industry combined, introduced 5G services in 2019 and will ensure that Stamford Bridge is 5G-ready within the first year of the collaboration with the Chelsea club.
Aside from Chelsea, Ireland is another footballing nation that uses the three logos on their jersey. Though, Ireland jerseys that contain the logo are not used during football matches.