if, as SAKryukov states, you simply want to define a constantant 3rd indexer then your method should just take 3 ints for indexer values and you can pass your sonstant to the third,
However, on the offchance that you are actually talking about jagged arrays (an array of arrays of arrays of values, for a 3D jagged array) you could try an extension method on array, something along the lines of
public static T[][][] AddConstantDimension<t>(this T[][] array, int extraDimensionLength, T extraVal)
{
T[][][] result = new T[][][] {};
for (int i = 0; i < array.GetLength(0); i++)
{
T[][] inner = new T[][] { };
for (int j = 0; j < array.GetLength(1); j++)
{
T[] inner2 = new[] {extraVal};
inner[j] = inner2;
}
result[i] = inner;
}
return result;
}
</t>
Then your call to your method would be
someObject.Method(2dJaggedArray.AddConstantDimension(5,2))
which would add a 3rd dimension, of length 5, with the value 2 inserted
NB: none of the above is tested, but should provide at least a starting point