Okay, so I messed around with your suggestion, and came up with this:
If acct_num <> Account1 Then
Account2.Value = DLookup("acct_num", "[AMS_Accounts]", "acc_lnk_id=" & Link_ID)
End If
If acct_num <> Account2 Or Account1 Then
Account3.Value = DLookup("acct_num", "[AMS_Accounts]", "acc_lnk_id=" & Link_ID)
End If
However, I end up getting duplicates through it:
Account1: 12345678
Account2: 98765432
Account3: 98765432
And sometimes, depending on what account number is entered first, all three accounts could be the same.