| 3I Group PLC | London | 1973 |
| 3I IIF GP 2020 Limited | London | 2020 |
| 3I Infra GP 2022 (Scots) Limited | Edinburgh | 2022 |
| 3I International Services PLC | London | 2001 |
| 3I Investments PLC | London | 2000 |
| 3I IP Acquisitions Limited | London | 2020 |
| 3I Managed Infrastructure Acquisitions GP Limited | London | 2016 |
| 3I MIA Holdings Limited | London | 2017 |
| 3I MIA II GP (2026) Limited | London | 2026 |
| 3I NAI Holdings GP Limited | Edinburgh | 2022 |
| 3I Nominees Limited | London | 1994 |
| 3I Nordic PLC | London | 2001 |
| 3I Osprey GP Limited | London | 2007 |
| 3I Oystercatcher Holdco Limited | London | 2025 |
| 3I PLC | London | 1945 |
| 3I SCI Holdings Limited | London | 2018 |
| 3I Tampnet Holdings Limited | London | 2019 |
| 3I Zephyr GP (2022) Limited | London | 2022 |
| 3IMEDICINE Ltd | Leicester | 2023 |
| 3IN Attero Holdco Limited | London | 2020 |
| 3IN MIA II Holdco Limited | London | 2026 |
| 3J7 Limited | Hailsham | 2024 |
| 3JA Holdings Ltd | Irlam | 2023 |
| 3JV Holdings Ltd | Luton | 2024 |
| 3K Group Ltd | Leicester | 2026 |
| 3K Holdings Investments UK Limited | London | 2025 |
| 3K Investments Limited | Windsor | 2022 |
| 3KSE Ltd | Epsom | 2023 |
| 3L Care Holdings Limited | Cheltenham | 2016 |
| 3L Risk Management Ltd | Haslemere | 2024 |
| 3L Security Services Limited | Richmond | 2025 |
| 3LB Limited | London | 2025 |
| 3LB Partners Ltd | Basingstoke | 2019 |
| 3LB Properties Ltd | Basingstoke | 2019 |
| 3LD Group Ltd | Maidstone | 2025 |
| 3LINE Holdings Limited | Poole | 2015 |
| 3LMK Limited | Birmingham | 2020 |
| 3LW Group Ltd | Limavady | 2024 |
| 3M Asia Pacific UK Holding Ltd | Bracknell | 2015 |
| 3M Financial Ltd | Wakefield | 2026 |
| 3M Iberia Trading Ltd | London | 2023 |
| 3M Products Limited | Berkshire | 1973 |
| 3M UK Holdings Limited | Berkshire | 1929 |
| 3MB Limited | Cleckheaton | 2020 |
| 3MC (U.K.) Limited | Altrincham | 1998 |
| 3MG International Limited | London | 2021 |
| 3MS Concept Ltd | London | 2025 |
| 3MSE Holdings Ltd | London | 2026 |
| 3MSKTRS Holdings Ltd | Glasgow | 2024 |
| 3N Capital Limited | St. Leonards-On-Sea | 2022 |
| 3N Management Ltd | London | 2013 |
| 3NV Investments Unlimited | London | 2023 |
| 3OL Group Ltd | Chester | 2026 |
| 3ONE6 Solutions Limited | Chorley | 2010 |
| 3OU Holdings Limited | Leeds | 2022 |
| 3P Holdings Limited | North Yorkshire | 2003 |
| 3PILLAR Associates Ltd | Ringwood | 2025 |
| 3PL Group Limited | Wigan | 2016 |
| 3PLI Limited | Henley-On-Thames | 2021 |
| 3POINT Holdings Ltd | Battle | 2014 |
| 3PX Holdings Limited | London | 2023 |
| 3Q Financial Limited | Bordon | 2020 |
| 3R Financial Advisers Ltd | Lincoln | 2020 |
| 3R Financial Services Ltd | Lincoln | 2013 |
| 3R Holdings Limited | Southampton | 2016 |
| 3R Incorporated Limited | Edinburgh | 2026 |
| 3R Payroll Limited | Hull | 2016 |
| 3R Solutions (Holdings) Limited | Edinburgh | 2014 |
| 3RD Chapter Wealth Management Ltd | London | 2025 |
| 3RD EYE VC 1 Limited | London | 2016 |
| 3RD EYE VC Limited | London | 2016 |
| 3RD Light Holdings Limited | Reading | 2018 |
| 3RD March Limited | Manchester | 2022 |
| 3RDSTAGE Ventures Ltd | Maidenhead | 2025 |
| 3RE (11BST) Midco Limited | London | 2025 |
| 3RE (33SJS) Midco Limited | London | 2026 |
| 3RIPLE N Limited | Northampton | 2013 |
| 3S - Solar Storage Systems Limited | London | 2024 |
| 3S Hassanali Limited | Coventry | 2026 |
| 3S Holdings Limited | Maidenhead | 2024 |
| 3S London Group Limited | Pinner | 2022 |
| 3S Northampton (Holdings) Limited | Northampton | 2022 |
| 3S PAY Limited | London | 2024 |
| 3SB Investment Holdings Limited | Stockport | 2016 |
| 3SEARCH Holdings Limited | Cheltenham | 2022 |
| 3SIXTY Financial Limited | Sunbury-On-Thames | 1998 |
| 3SIXTY Financial Solutions Limited | Sheffield | 2013 |
| 3SIXTY Investment Holdings Limited | Watchfield Swindon | 2021 |
| 3SKH Limited | Milton Keynes | 2025 |
| 3SM Investments Limited | Darlington | 2019 |
| 3T Bros Limited | Cardiff | 1997 |
| 3T Capital | Wolsingham | 2019 |
| 3T Finance Solutions Ltd | Wokingham | 2012 |
| 3TAU OM Ltd | London | 2025 |
| 3TC Holdings Limited | London | 2009 |
| 3TP Limited | Auchterarder | 2022 |
| 3UNDER4 Investments Ltd | St. Albans | 2025 |
| 3V Holdings Limited | Nottingham | 2019 |
| 3VI 4.5 Acres Ltd | London | 2022 |
| 3VO Ltd | Lowestoft | 2024 |