I don't remember exactly - but level 3 care is slightly different from being a level 3 unit. The COC is a level 2 neonatal unit (or Local Neonatal Unit, LNU) but can provide some level 3 care. https://www.bliss.org.uk/parents/in-hospital/about-neonatal-care/how-does-neonatal-care-work
Level 3 care, in ICU terms, means anyone on a ventilator via a tube in their throat (not CPAP via a mask), or anyone requiring multi-organ support. A level 2 unit, like the COC, as I understand it (doctor with experience in adult ICU but not NICU) is able to provide some short term level 3 care for babies needing ventilation etc - generally only for a couple of days. If they were likely to need longer term ventilation, or be deteriorating, they would aim to transfer out. Generally level 2 units only take babies from ~28 weeks - though if an earlier baby were born there unexpectedly, they would stabilise them and aim to transfer out.
A level 3 neonatal unit cares for all extreme preterm (<28w) infants, babies needing invasive ventilation for longer than a few days, multiple organ support, and babies needing surgery. There are some even more specialist hospitals looking after babies needing e.g. cardiac surgery shortly after birth - only a handful of these in the UK.