| 37/39A Cathedral Road Holdings Ltd | Cardiff | 2018 |
| 371 Kennington Lane Ltd | London | 2023 |
| 377 Holdings Limited | Leigh On Sea | 2022 |
| 37934 Crowd Limited | Exeter | 2026 |
| 37LONDON Group Ltd | London | 2021 |
| 38 West Consultants Ltd | London | 2024 |
| 381 Holdings Limited | London | 2020 |
| 381 Investments Limited | Chatteris | 2010 |
| 39 Hale Grove Gardens Limited | Wembley | 2018 |
| 39 Moreland Street Ltd | London | 2025 |
| 393 Ltd | London | 2024 |
| 3973 Ltd | Mildenhall | 2024 |
| 3:33 Group Limited | London | 2025 |
| 3A Asset Group Ltd | Bradford | 2025 |
| 3A Greek Properties Limited | London | 2024 |
| 3ACE Construction Ltd | London | 2014 |
| 3ADR Limited | London | 2023 |
| 3AK Limited | Bradford | 2013 |
| 3AM Holdings Ltd | Poole | 2022 |
| 3AMIGOSTRADING Ltd | Farnham | 2025 |
| 3AV Limited | London | 2026 |
| 3AVA Holdings Ltd | Atherstone | 2024 |
| 3B Investment Group Ltd | Glasgow | 2019 |
| 3BLOCKS Ltd | London | 2015 |
| 3BY7 Limited | Liverpool | 2021 |
| 3C Advisors Ltd | London | 2023 |
| 3C Financial Services Limited | Newcastle | 2019 |
| 3C International Property Investments Limited | Eastbourne | 2009 |
| 3C Payment UK Ltd | London | 1996 |
| 3CAD Management Ltd | Rochester | 2026 |
| 3COM (UK) Limited | London | 2024 |
| 3CS Finance Limited | New Malden | 2024 |
| 3CSB Ltd | London | 2026 |
| 3CV Holdings Ltd | London | 2022 |
| 3CY Ltd | Worcester Park | 2023 |
| 3D Analytics Ltd | Greenhithe | 2014 |
| 3D Capital & Advisory Ltd | London | 2014 |
| 3D Capital Holdings Ltd | Margate | 2025 |
| 3D Capital Investments Ltd | Harrow | 2021 |
| 3D Capital Limited | London | 2012 |
| 3D Financial Planning Ltd | Reigate | 2012 |
| 3D Group Ltd | Milford Haven | 2020 |
| 3D Homes Group Limited | London | 2024 |
| 3D Lipo Holdings Ltd | London | 2016 |
| 3D Lipo Investments Ltd | London | 2024 |
| 3D Mortgages Ltd | Lincoln | 2021 |
| 3D Protective Group Ltd | Manchester | 2025 |
| 3D Recruit (Holdings) Ltd | Battle | 2017 |
| 3D Rehabilitation Limited | Bolton | 2007 |
| 3D Wealth Management Ltd | Tarporley | 1997 |
| 3D Wealth Partners Limited | Towcester | 2010 |
| 3DI Group Ltd | Ayr | 2011 |
| 3DIFS Limited | Coventry | 2006 |
| 3DINVEST Group Ltd | Harrow | 2020 |
| 3DMT Ltd | Buckhurst Hill | 2019 |
| 3DO Holdings Ltd | Bury | 2026 |
| 3DOM Group Holdings Ltd | Leighton Buzzard | 2022 |
| 3DOT Holdings Limited | Newton Abbot | 2020 |
| 3DS Financial Services Ltd | Cambridge | 2015 |
| 3DTD Holdings Limited | Milton Keynes | 2019 |
| 3E Motors Ltd | Harrogate | 2017 |
| 3ET Holdings Limited | Markfield | 2021 |
| 3EYES Capital Services Ltd | London | 2023 |
| 3F Capital Limited | Aberdeen | 2025 |
| 3FACTORHOLDINGS UK Bidco Limited | London | 2020 |
| 3FACTORHOLDINGS UK Topco Limited | London | 2020 |
| 3FIVE1 Financial Limited | Coventry | 2018 |
| 3FR Group Ltd | London | 2022 |
| 3FS Capital Ltd | London | 2021 |
| 3G Capital Investments Ltd | London | 2021 |
| 3G Holdco Limited | Rotherham | 2021 |
| 3G Holdings Ltd | Peterborough | 2025 |
| 3G Tunnelling Services Limited | Harrow | 2011 |
| 3G Winnard Limited | Rotherham | 2025 |
| 3GEN Group Ltd | Oldham | 2026 |
| 3GS Holdings Ltd | Manchester | 2015 |
| 3H Investments Limited | Edinburgh | 2002 |
| 3H Private Capital Limited | London | 2026 |
| 3H20 Capital Ltd | London | 2003 |
| 3HIVE Holdings Limited | Poole | 2022 |
| 3HN Homes Limited | London | 2021 |
| 3HR Ltd | Birmingham | 2023 |
| 3I Amalthea Topco Limited | London | 2021 |
| 3I Aura GP (2022) Limited | London | 2022 |
| 3I EFV GP Limited | London | 2006 |
| 3I EFV Nominees a Limited | London | 2006 |
| 3I EFV Nominees B Limited | London | 2006 |
| 3I Europe PLC | London | 1993 |
| 3I European Mid-Market Infrastructure GP (2024) Limited | London | 2024 |
| 3I GC GP Limited | London | 2009 |
| 3I GC Nominees a Limited | London | 2010 |
| 3I GC Nominees B Limited | London | 2010 |
| 3I GP 2010 Limited | London | 2010 |
| 3I GP 2013 Ltd | London | 2013 |
| 3I GP 2016 Limited | London | 2016 |
| 3I GP 2019 (Scots) Limited | Edinburgh | 2019 |
| 3I GP 2019 Limited | London | 2019 |
| 3I GP 2020 Limited | Edinburgh | 2019 |
| 3I GP 2022 (Scots) Limited | Edinburgh | 2022 |
| 3I GP 2022 Limited | London | 2022 |