You could use LINQ like so:
' VB.NET
Dim str = "123abc456def789ghij"
Dim len = 3
Dim arr = Enumerable.Range(0, str.Length / len).Select (Function(x) str.Substring(x * len, len)).ToArray()
// C#
var str = "123abc456def789ghij";
var len = 3;
var arr = Enumerable.Range(0, str.Length / len).Select (x => str.Substring(x * len, len)).ToArray();
Note this will only take complete occurrences of length (i.e. 3 sets in a string 10 characters long).
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