| 3G Construction Consultants Limited | Waterbeach | 2012 |
| 3G Consultancy & Training Ltd | Aberdare | 2014 |
| 3G Resources Limited | Brighouse | 2014 |
| 3G Winnard Trustees Limited | Rotherham | 2025 |
| 3G4G Limited | Ilford | 2016 |
| 3GAAPS Ltd | London | 2021 |
| 3GEN Water Ltd | Sale | 2020 |
| 3GL Events Limited | Tadworth | 2023 |
| 3GLOBAL TOU Ltd | London | 2026 |
| 3H Freehold Limited | London | 2002 |
| 3H Learning Solutions Ltd | Stone | 2023 |
| 3H2O Ltd | London | 2025 |
| 3HFA Limited | Bolton | 2026 |
| 3HM Engineering Limited | Warrington | 2016 |
| 3HN Limited | London | 2013 |
| 3HREE 2WO Creative Ltd | London | 2022 |
| 3HW Architecture and Design Limited | Helston | 2012 |
| 3HW Property Limited | Plymouth | 2012 |
| 3I Communications Ltd | Horsham | 2012 |
| 3I Condition Monitoring Consultancy Limited | Stockport | 2006 |
| 3I Consulting Ltd | Preston | 2011 |
| 3I Holdings PLC | London | 1991 |
| 3I International Holdings | London | 1945 |
| 3I Management Limited | Laurencekirk | 2017 |
| 3I Marketing Services Limited | Rowhedge | 2017 |
| 3I Technical Consulting Ltd | Shefford | 2019 |
| 3I Technology Company Ltd | St. Asaph | 2021 |
| 3I Training & Consultancy Limited | Glasgow | 2015 |
| 3IC Solutions Limited | London | 2019 |
| 3ICE Ltd | Glasgow | 2023 |
| 3ILM Wireless Limited | Watford | 2019 |
| 3IM Limited | Slough | 2019 |
| 3IN Consulting Ltd | Glasgow | 2017 |
| 3IVE Limited | London | 2025 |
| 3J (2003) Ltd | Crewe | 2003 |
| 3J Consulting Limited | Alfreton | 2024 |
| 3J Hunter Limited | Port Glasgow | 2018 |
| 3J Site Services Limited | Liverpool | 2024 |
| 3J2 Business Consulting Ltd | Sale | 2024 |
| 3JCR Limited | Newry | 2023 |
| 3JD Consultants Ltd | Dereham | 2022 |
| 3JE Consulting Limited | Poole | 2020 |
| 3JE.ENGINEERING Consultant Ltd | Orpington | 2023 |
| 3JG Limited | London | 2025 |
| 3JNOBLE Management Services Ltd | London | 2018 |
| 3JP Consulting Ltd | Ilkeston | 2022 |
| 3JS Consulting Limited | Harpenden | 2013 |
| 3K + V Limited | Harrow | 2002 |
| 3K Accounting (Leeds) Ltd | Rugby | 2026 |
| 3K Accounting (Southampton) Ltd | Rugby | 2025 |
| 3K Accounting (Yorkshire) Ltd | Rugby | 2026 |
| 3K Accounting Ltd | Rugby | 2025 |
| 3K Consulting Ltd | Warwickshire, | 2026 |
| 3K Financial & Accounting Services Ltd | London | 2022 |
| 3K Law Ltd | Oldham | 2017 |
| 3K0 Architectural Design Ltd | London | 2021 |
| 3KAK Ltd | Northwood | 2022 |
| 3KAP.X Limited | Slough | 2025 |
| 3KEEL Group Ltd | York | 2022 |
| 3KLEGEND Ltd | Warrington | 2026 |
| 3KM Global Sport Ltd | Warrington | 2026 |
| 3KONCEPT Ltd | Kingston Upon Thames | 2024 |
| 3KQ Limited | Kendal | 2004 |
| 3KS Consultancy Ltd | Harrow | 2021 |
| 3KS Partnership Ltd | Crewe | 2014 |
| 3L Engineering Solutions Ltd | Newcastle Upon Tyne | 2026 |
| 3L Media Limited | London | 2018 |
| 3L Ventures Limited | London | 2023 |
| 3LA Studio Ltd | London | 2020 |
| 3LAN Ltd | London | 2026 |
| 3LANCE Marketing Limited | Birmingham | 2025 |
| 3LB Consulting Ltd | Harrogate | 2024 |
| 3LD Coaching Ltd | York | 2022 |
| 3LISTR Limited | Morecambe | 2019 |
| 3LKA Interpreting Limited | Crawley | 2026 |
| 3LM Services Ltd | Keston | 2025 |
| 3LOD Advisory Ltd | Holyhead Road | 2026 |
| 3LOD Consulting Ltd | York | 2024 |
| 3LOD Limited | London | 2019 |
| 3LOD Risk Management Services Limited | Newtownards | 2015 |
| 3LR Limited | Worcester | 2023 |
| 3LUX2 Ltd | Portsmouth | 2016 |
| 3M Accountants Ltd | Smethwick | 2020 |
| 3M Advisory Limited | Shipston-On-Stour | 2025 |
| 3M Consultancy Group Ltd | Huddersfield | 2022 |
| 3M Cubed Solutions Ltd | Aberdeen | 2014 |
| 3M Design Ltd. | Bournemouth | 2018 |
| 3M Health Care Limited | London | 1969 |
| 3M Jelley Consultancy Limited | London | 2015 |
| 3M Law Limited | Leeds | 2019 |
| 3M Legal Ltd | Halifax | 2025 |
| 3M Pension Trustees Limited | Berkshire | 1979 |
| 3M RPS Ltd | London | 2019 |
| 3M Services U.K. Limited | Smethwick | 2025 |
| 3M Solutions Ltd | Solihull | 2026 |
| 3M'S Corporate Services Limited | Warrington | 2022 |
| 3MA Holdings Limited | Birmingham | 2019 |
| 3MAN Enterprises Ltd | London | 2018 |
| 3MARINE Limited | Stonehaven | 2014 |
| 3MATTER Ltd | Cardiff | 2016 |