Samsung announced the start of the One UI 3 beta program based on Android 11 earlier today for the Galaxy M31 and now it’s doing the same for the Galaxy A51 5G. It was announced in the Samsung Community forums in South Korea.
You can check if you are eligible to enter the program by tapping on the banner in the Samsung Members app to sign up. A beta version of the software will be seeded to you shortly after. According to the European Android 11 release schedule, the Galaxy A51 5G is supposed to get the stable version of the software in March.
Source 1 (in Korean)