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Improved version of your function (using C++17 std::string_view as the Parameter to avoid any copies, even in the caller, whether he has a std::string or not):

Improved version of your function (using C++17 std::string_view as the parameter to avoid any copies, even in the caller, whether he has a std::string or not):

Making copies, reversing, and is just superfluous additional work.

Improved version of your function (using C++17 std::string_view to avoid any copies):

long substrCount(std::string_view s) noexcept {
    char c[3] = {};
    long n[3] = {};
    long r = 0;
    for (auto curr = begin(s), last = end(s), pos = begin(s); curr != last; curr = pos) {
        pos = std::find_if(curr + 1, last, [=](char c){ return c != *curr; });
        n[2] = n[1];
        n[1] = n[0];
        n[0] = pos - curr;
        c[2] = c[1];
        c[1] = c[0];
        c[0] = *curr;
        r += *n * (*n + 1) / 2;
        if (n[1] == 1 && c[0] == c[2])
            r += std::min(n[0], n[2]);
    }
    return r;
}

Making copies, reversing, and then comparing is just superfluous additional work.

Improved version of your function (using C++17 std::string_view as the Parameter to avoid any copies, even in the caller, whether he has a std::string or not):

long substrCount(std::string_view s) noexcept {
    char c[3] = {};
    long n[3] = {};
    long r = 0;
    for (auto curr = begin(s), last = end(s), pos = begin(s); curr != last; curr = pos) {
        pos = std::find_if(curr + 1, last, [=](char c){ return c != *curr; });
        std::tie(n[2], n[1], n[0]) = std::make_tuple(n[1], n[0], pos - curr);
        std::tie(c[2], c[1], c[0]) = std::make_tuple(c[1], c[0], *curr);
        r += *n * (*n + 1) / 2;
        if (n[1] == 1 && c[0] == c[2])
            r += std::min(n[0], n[2]);
    }
    return r;
}

long substrCount(std::string_view s) noexcept {
    char c[3] = {};
    long n[3] = {};
    long r = 0;
    auto count = [&]{
        r += *n * (*n + 1) / 2;
        if (n[1] == 1 && c[0] == c[2])
            r += std::min(n[0], n[2]);
    };
    for (auto x : s) {
        if (x == *c) {
            ++*n;
        } else {
            count();
            c[2] = c[1];
            c[1] = c[0];
            c[0] = x;
            n[2] = n[1];
            n[1] = n[0];
            n[0] = 1;
        }
    }
    count();
    return r;
}

long substrCount(std::string_view s) noexcept {
    char c[3] = {};
    long n[3] = {};
    long r = 0;
    for (auto curr = begin(s), last = end(s), pos = begin(s); curr != last; curr = pos) {
        pos = std::find_if(curr + 1, last, [=](char c){ return c != *curr; });
        n[2] = n[1];
        n[1] = n[0];
        n[0] = pos - curr;
        c[2] = c[1];
        c[1] = c[0];
        c[0] = *curr;
        r += *n * (*n + 1) / 2;
        if (n[1] == 1 && c[0] == c[2])
            r += std::min(n[0], n[2]);
    }
    return r;
}