'F4BC を１文字として認識してるね。バイト操作ならバイト型がいいでしょう
Private Sub プログラムはこんな感じです()
Dim FileNo1 As Integer
Dim FileNo2 As Integer
Dim rb(1 To 3) As Byte
Dim wb(1 To 3) As Byte
Dim I As Byte
FileNo1 = FreeFile
Open "c:\temp\a.tmp" For Binary Access Read As #FileNo1
FileNo2 = FreeFile
Open "c:\temp\b.tmp" For Binary Access Write As #FileNo2
Get #FileNo1, 1, rb
For I = 1 To 3
wb(4 - I) = rb(I)
Next I
Put #FileNo2, , wb
End Sub