C++ :

#include<iostream>
#include<fstream>
#include<algorithm>
#include<string>
#include<cstring>
using namespace std;
const int N=101;
string s1,s2,s3;
string F[N][N][N];char ff[5][5][5];
inline bool f(int i,int j,int k){return s1[i-1]==s2[j-1]&&s2[j-1]==s3[k-1];}
inline const string& maxs(const string &a,const string &b){return (a.size()>b.size())?a:b;}
int main(){
    ios::sync_with_stdio(0);
    cin>>s1>>s2>>s3;
    int n1,n2,n3;
    n1=s1.size();n2=s2.size();n3=s3.size();
    for(int i=1;i<=4;i++)
        for(int j=1;j<=4;j++)
            for(int k=1;k<=4;k++){
                if(f(i,j,k))F[i][j][k]=F[i-1][j-1][k-1]+s1[i-1];
                else F[i][j][k]=maxs(maxs(F[i][j][k-1],F[i][j-1][k]),F[i-1][j][k]);
            }
    for(int i=1;i<=n1;i++)
        for(int j=1;j<=n2;j++)
            for(int k=1;k<=n3;k++){
                if(F[i][j][k]=="")continue;
                memset(ff,0,sizeof(ff));
                for(int ai=1;ai<=4&&i+ai<=n1;ai++)
                    for(int aj=1;aj<=4&&j+aj<=n2;aj++)
                        for(int ak=1;ak<=4&&k+ak<=n3;ak++){
                            if(f(ai+i,aj+j,ak+k))ff[ai][aj][ak]=s1[ai+i-1];
                            else if(ff[ai-1][aj][ak])ff[ai][aj][ak]=ff[ai-1][aj][ak];
                            else if(ff[ai][aj-1][ak])ff[ai][aj][ak]=ff[ai][aj-1][ak];
                            else if(ff[ai][aj][ak-1])ff[ai][aj][ak]=ff[ai][aj][ak-1];
                        }
                for(int ai=1;ai<=4&&i+ai<=n1;ai++)
                    for(int aj=1;aj<=4&&j+aj<=n2;aj++)
                        for(int ak=1;ak<=4&&k+ak<=n3;ak++)
                            if(ff[ai][aj][ak])
                                F[i+ai][j+aj][k+ak]=F[i][j][k]+ff[ai][aj][ak];
            }
    cout<<F[n1][n2][n3]<<endl;
    return 0;
}