How can I concatenate multiple byte Arrays into one Array
You can concatenate with memcpy. You just need to set the pointer at the right place inside the c1 array.

memcpy(c1, a1, sizeof(a1));
memcpy(c1+sizeof(a1), a2, sizeof(a2));
memcpy(c1+sizeof(a1)+sizeof(a2), a3, sizeof(a3));