三维的与二维大同小异，看代码。
#include<cstdio>
#include<cstring>
#include<algorithm>
#define REP(i, a, b) for(int i = (a); i < (b); i++)
using namespace std;const int MAXN = 112;
char a[MAXN], b[MAXN], c[MAXN];
int f[MAXN][MAXN][MAXN], path[MAXN][MAXN][MAXN];void print(int x, int y, int z)
{int t = path[x][y][z];if(t == 1){print(x - 1, y - 1, z - 1);putchar(a[x]);}else if(t == 2) print(x - 1, y, z);else if(t == 3) print(x, y - 1, z);else if(t == 4) print(x, y, z - 1);
}int main()
{scanf("%s%s%s", a + 1, b + 1, c + 1);int lena = strlen(a + 1), lenb = strlen(b + 1), lenc = strlen(c + 1);REP(i, 1, lena + 1)REP(j, 1, lenb + 1)REP(k, 1, lenc + 1){if(a[i] == b[j] && b[j] == c[k]) {f[i][j][k] = f[i-1][j-1][k-1] + 1;path[i][j][k] = 1;}else {f[i][j][k] = max(f[i-1][j][k], max(f[i][j-1][k], f[i][j][k-1]));if(f[i][j][k] == f[i-1][j][k]) path[i][j][k] = 2;if(f[i][j][k] == f[i][j-1][k]) path[i][j][k] = 3;if(f[i][j][k] == f[i][j][k-1]) path[i][j][k] = 4;}}printf("%d\n", f[lena][lenb][lenc]);print(lena, lenb, lenc);return 0;
}