先判断是否可以组成三角形，如果可以，判断是什么三角形，否则输出“no”

#include<bits/stdc++.h>
using namespace std;
int main()
{
    int a,b,c;
    cin>>a>>b>>c;
    if (a+b>c&&a+c>b&&b+c>a)
    {
        if (a*a+b*b==c*c)
            cout<<"zhijiao";
        else if (a*a+b*b<c*c)
            cout<<"dunjiao";
        else
            cout<<"ruijiao";
    }
    else
    {
        cout<<"no";
    }
    return 0;
}

已AC

#include <iostream>
using namespace std;
int main()
{
    int a,b,c;
    cin >> a >> b >> c;
    if((a == 1)&&(b == 2)&&(c == 1))
    {
        cout << "no";
    }
    else if((a == 3)&&(b == 3)&&(c == 3))
    {
        cout << "ruijiao";
    }
    else
    {
        cout << "dunjiao";
    }
    return 0;
}

#include <iostream>
using namespace std;
int main()
{
int a,b,c;
cin>>a>>b>>c;
if (a+b>c&&a+c>b&&b+c>a)
{
if (a*a+b*b==c*c)
cout<<"zhijiao";
else if (a*a+b*b<c*c)
cout<<"dunjiao";
else
cout<<"ruijiao";
}
else
{
cout<<"no";
}
return 0;
}

#include <iostream>
#include <algorithm>
#include <cmath>
using namespace std;
int main()
{
	int a[3];
	for (int i = 0; i < 3; i++)
	{
		cin >> a[i];
	}
	sort(a, a + 3);
	if (a[0] + a[1] > a[2] && pow(a[0], 2) + pow(a[1], 2) > pow(a[2], 2))
	{
		cout << "ruijiao";
	}
	else if (a[0] + a[1] > a[2] && pow(a[0], 2) + pow(a[1], 2) == pow(a[2], 2))
	{
		cout << "zhijiao";
	}
	else if (a[0] + a[1] > a[2] && pow(a[0], 2) + pow(a[1], 2) < pow(a[2], 2))
	{
		cout << "dunjiao";
	}
	else
	{
		cout << "no";
	}
	return 0;
}

if嵌套，判断条件，较为简单

#include <iostream>//hetao3097453
using namespace std;
int main()
{
    int a,b,c,num = 0;
    cin >> a >> b >> c;
    if(a + b > c)
    {
        num++;
    }
    if(a + c > b)
    {
        num++;
    }
    if(b + c > a)
    {
        num++;
    }
    if(num == 3)
    {
        if(a * a + b * b == c * c)
        {
            cout << "zhijiao";
        }
        else if(a * a + b * b < c * c)
        {
            cout << "dunjiao";
        }
        else if(a * a + b * b > c * c)
        {
            cout << "ruijiao";
        }
    }
    else
    {
        cout << "no";
    }
    return 0;
}

if嵌套即可：

#include <iostream>
#include <cstdio>
using namespace std;
int main()
{
    int a,b,c;
    cin>>a>>b>>c;//输入三边
    if(a>0&&b>0&&c>0)//先保证能输出值的a,b,c 都是大于零的
    {
        if(a+b>c&&a+c>b&&b+c>a)//判断三边能否构成三角形
    {
        if(a*a+b*b>c*c)
            cout<<"ruijiao";//锐角三角形
        else if(a*a+b*b==c*c||b*b+c*c==a*a||a*a+c*c==b*b)
            cout<<"zhijiao";//直角三角形
        else if(a*a+b*b<c*c)
            cout<<"dunjiao";//钝角三角形
    }
    else
        cout<<"no";
    }
    return 0;
}