C语言的三种基本程序结构是什么

#include<stdio.h>
int main()
{
    char x,y;  
    
    scanf("%c",&x);
    
    if(x >= &#39;A&#39; && x <= &#39;Z&#39;)
    { 
        y=x+32;
    }
    
    else
    {
        printf("this is a erro"); 
    }
    printf("%c\n",y);
    return 0;
}

#include<stdio.h>
int main()
{
    float a,b,c,tmp;
   
    scanf("%f %f %f",&a,&b,&c);
    if(a > b)
    {
        tmp=b;
        b=a;
        a=tmp;
    }
    if(a > c)
    {
        tmp=c;
        c=a;
        a=tmp;
    }          
    if(b > c)
    {
        tmp=c;
        c=b;
        b=tmp;
    }        
       
    printf("%5.2f %5.2f %5.2f\n",a,b,c);
    return 0;
}

#include<stdio.h>
int main()
{   
    int i,x=1;
    for(i = 2;i <= 100;i++)
    {
        x+=i;
    }
    printf("%d\n",x);
    return 0;
}