Adding object variables from a list

This overload of System.Linq.Enumerable.Sum should work nicely for you:

Computes the sum of the sequence of Int32 values that are obtained by invoking a transform function on each element of the input sequence.

In this case, the "transform function" would be a Func<TSource, Int32> that returns the value of the variable3 property for the item (e.g. obj => obj.variable3).

You could use it like this:

int TotalObjectVarN3 = new []{ Object1, Object2, Object3 }.Sum(obj => obj.variable3);

This one does it using LINQ

int TotalObjectVarN3 = MyList.Sum(o => o.variable3):

Using Select is an option followed by the Sum method. Select Projects each element of a sequence into a new form.

list.Select(i => i.variable3).Sum();

More about Select - https://docs.microsoft.com/en-us/dotnet/api/system.linq.enumerable.select?view=net-5.0