code: func NonBlockingWrite3(fd int, b1, b2, b3 []byte) *tcpip.Error {
  	// If the is no second buffer, issue a regular write.
  	if len(b2) == 0 {
  		return NonBlockingWrite(fd, b1)
  	}
  
  	// We have two buffers. Build the iovec that represents them and issue
  	// a writev syscall.
  	iovec := [3]syscall.Iovec{
  		{
  			Base: &b1[0],
  			Len:  uint64(len(b1)),
  		},
  		{
  			Base: &b2[0],
  			Len:  uint64(len(b2)),
  		},
  	}
  	iovecLen := uintptr(2)
  
  	if len(b3) > 0 {
  		iovecLen++
  		iovec[2].Base = &b3[0]
  		iovec[2].Len = uint64(len(b3))
  	}
  
  	_, _, e := syscall.RawSyscall(syscall.SYS_WRITEV, uintptr(fd), uintptr(unsafe.Pointer(&iovec[0])), iovecLen)
  	if e != 0 {
  		return TranslateErrno(e)
  	}
  
  	return nil
  }
docs: // NonBlockingWrite3 writes up to three byte slices to a file descriptor in a
  // single syscall. It fails if partial data is written.