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;
}

C++ :

#include <iostream>

using namespace std;

string check(string a, int index, int size) {
	size += index;
	size = size > a.size() ? a.size() : size;
	string r = "";
//	cout << "from: " << index << " to: " << size << endl;
	for (; index < size; index++) {
		r += a[index];
	}
//	cout << "get: " << r << endl;
	return r;
}

int main() {
	string msg, a, b;
	getline(cin, msg);
	msg += ' ';
	getline(cin, a);
	getline(cin, b);
	string c = "";
	for (int i = 0; i < msg.size(); i++) {

		if (a[0] == msg[i] and check(msg, i, a.size()) == a ) {
//			cout << "get:" << a << " " << a.size() << endl;
			c += b;
			i += a.size() - 1;
		} else
			c += msg[i];
	}
	cout << c;

}

Python :

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