1.Ber
2.Dub
3.Lub
4.Osk
5.Sta
6.Tal

The list above consists of 6 triplets of letters, which you are asked to partition into 3 subsets:
Subset I will contain 4 members sharing a common feature,
Subset II will contain one member that can be added to Set I, if its definition is broadened,
Subset III, the last triplet that is "odd man out"

Good luck!