Count unique records against 3 columns

we would understand it in this direction

group the data on colA, colB
From the group members take the last member (as we do see NOK and not OK)

5699510 information OK
5699510 information NOK

dt2 =

(From d in dt1.AsEnumerable
Group d by k1 = d("ColA").toString.Trim,k2 = d("ColB").toString.Trim into grp=Group
Select r = grp.Last()).CopyToDaTable

Modify ColA, ColB name or index to your details

Also have a look here: