Java 7, 148 bytes

String c(){String r,z,y=z=r="",p="/////\\\\\\\\\\",q="\\\\\\\\\\/////";int i=0;for(;i++<6;y+=p,z+=q);for(i=0;i<24;r+=(i++%8<4?y:z)+"\n");}return r;}

Ungolfed & test code:

Try it here.

class M{
  static String c() {
    String r, z, y = z = r = "",
           p = "/////\\\\\\\\\\",
           q = "\\\\\\\\\\/////";
    int i = 0;
    for(; i++ < 6; y += p, z += q);
    for(i = 0; i < 24; r += (i++%8 < 4 ? y : z)+"\n");
    return r;
  }

  public static void main(String[] a){
    System.out.println(c());
  }
}

Output:

/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\
/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\
/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\
/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\
\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////
\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////
\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////
\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////
/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\
/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\
/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\
/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\
\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////
\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////
\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////
\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////
/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\
/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\
/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\
/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\
\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////
\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////
\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////
\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////\\\\\/////

Java 11, 148 122 bytes

v->{String a="/////",b="\\\\\\\\\\",t=((a+b).repeat(5)+"\n").repeat(4),r=t+((b+a).repeat(5)+"\n").repeat(4);return r+r+r;}

Try it online.

Explanation:

v->{                       // Method with empty unused parameter & String return-type
  String a="/////",        //  Temp-String `a`, containing "/////"
         b="\\\\\\\\\\",   //  Temp-String `b`, containing "\\\\\"
         t=                //  Temp-String `t`, containing:
           ((a+b)          //   `a+b`
             .repeat(5)    //   Repeated 5 times
              +"\n")       //   With a newline
               .repeat(4), //   Repeated 4 times
         r=                //  Temp-String `r`, containing:
           t+              //   `t`, appended with:
             ((b+a).repeat(5)+"\n").repeat(4);
                           //    Same as above, for `b+a` instead of `a+b`
         return r+r+r;}    //  Return three `r`-Strings appended together

#Java 7, 148 bytes

Java 7, 148 bytes