HSPA+ isn't the same as DC-HSPA, though they'll likely both show as H+ in a status bar since DC-HSPA can be considered as an extension of HSPA+
DC-HSPA is similar to 4G carrier aggregation and uses multiple carriers to increase throughput.
@Zathan if Three have reduced their 3G down to a single carrier on 2100Mhz then its likely that they are at least planning to enable B1 4G, though usually that'd need a new radio at the site to do so.
Alternatively, there have also been reports of Three changing their network parameters over the past days/week so it could be that they messed something up and accidently did something that has caused a 3G carrier to no longer work.