C :

#include<stdio.h>
#include<string.h>
#include<ctype.h>
char s[1000],a[100],b[100],t[100],f[2];
int main(){
    gets(s);
    gets(a);
    gets(b);
    int i,j,n = strlen(s),m = strlen(a);
    for(i=0;i<n;){
        memset(t,0,sizeof(t)); 
        for(j=i;j<i+m && j<n;j++){
            f[0] = s[j];
            f[1] = '\0';
            strcat(t,f);
        }
        if(strcmp(t,a)==0){
            printf("%s",b); 
            i = j; 
        }else{
            printf("%c",s[i]);
            i++;
        }
    }
    return 0;
}

Python :

str1=input()
str2=input()
str3=input()
x=str1.replace(str2,str3)
print(x)