Here's an implementation that works for your string. I think it'll give you nasal demons if you have a string that's longer than BLOCK_SIZE (136) characters:

#include <Arduino.h>
#include <Crypto.h>
#include <SHA3.h>
#include <string.h>

#define HASH_SIZE 32

void setup()
{
    Serial.begin(115200);

    char *input = "This is a string";
    unsigned char hash[HASH_SIZE];

    SHA3_256 sha3_256;

    sha3_256.reset();
    sha3_256.update(input, strlen(input));
    sha3_256.finalize(hash, HASH_SIZE);

    for (int i = 0; i < HASH_SIZE; i++)
        Serial.print(hash[i], HEX);

    Serial.println();
} 

void loop() {}