def check_shuffle(s1, s2, s3):
    if len(s3) != len(s1) + len(s2):
        return False
    s1_pos = 0
    remains = ''
    for index, s in enumerate(s3):
        if s1_pos == len(s1):
            remains += s3[index:]
            break
        elif s1[s1_pos] == s:
            s1_pos += 1
        else:
            remains += s
    if s1_pos != len(s1):
        return False
    return remains == s2