Here's an example implementation in Python:

    def split_every_third(string):
        return string[::3][::-1]
    
    input_str = "This is a test"
    print(split_every_third(input_str))

Running this code should produce the output "s ssT" because it takes every third character (starting from index 0) from the original string, which would be "Tss s" and then reverses it to produce "s ssT".
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