To see the results simply run dotnet test
The logic for the bracket counting is in BracketService.cs
The logic for the multiple sum is in SumService.cs
I tested all of the cases specified in Tests.cs
For the sum of all the multiples of 3 or 5 below 1000 I got 233168, assuming you did NOT want to double count values that are divisible by both 3 and 5 (like 15 for example)