Three UK launched on March 3 2003 as a 3G-only carrier. Now fifteen years later, the company has decided to stop selling 3G phones and only offer LTE-enabled devices. Its the first carrier to make such a move.
The 3G network isnt going anywhere for now, this is just preparation for the future. The carrier needs to wait until most of its users have a 4G-capable phone and until its network improves. Threes LTE network launched in late 2013 and its not ready to fully replace its 3G network just yet.
The US switched off its 2G networks, but Vodafone UK has committed to keeping them alive until 2025. It actually might end up outliving 3G in Europe, so you dont need to worry. Still, its something to keep in mind when looking at buying new low-end devices (like the recent Nokia 3310 3G).
On the mainland thins are different. T-Mobile Czech Republic has plans to kill both its 2G and 3G networks by 2020. Vodafone Netherlands has similar plans. Telenor Norway will do the same by 2025 and rely on VoLTE instead. The spectrum currently used by those networks will be transferred to LTE and who knows, maybe even 5G.