Ответ:

1)

#include <iostream>

using namespace std;

int main() {

int n,m;

int sum=0;

cout << "Enter number n: ";

cin >> n;

cout << "Enter number m: ";

cin >> m;

for (int i = n; i <= m; i++) {

 if (i % 3 == 0) cout  << i<<endl;

}  

}

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2)

#include <iostream>

using namespace std;

int main() {

int n, m;

int sum = 0;

cout << "Enter number n: ";

cin >> n;

cout << "Enter number m: ";

cin >> m;

for (int i = n; i <= m; i++) {

 if (i % 2 != 0) sum+=i;

}

cout << "Result: " << sum << endl;

}

===================

3)

#include <iostream>

using namespace std;

int main() {

long n,m, max=0;

cout << "Enter number: ";

cin >> n;

m = n;

 for (int i = 10; n>0;) {

 if (n % i > max)  

 max = n % i;

 n = n / i;

}

 cout << "Result: " << max;

}

Объяснение: