Js solution:

return s.split('').reduce((accumulator ,letter, currentIndex: number) => {
    if(
        ((currentIndex%3 === 0 || currentIndex%3 === 2) && letter !== 'S') ||
        (currentIndex%3 === 1 && letter !== 'O')
    ){
        return accumulator += 1;
    }
    return accumulator;
}, 0);

}