Copy Row of Highlighted Cell

I noticed that there are occurrences of more than one highlighted cell in a row, so i am guessing that you want that row copied over only one...Give this a try...

Sub compareSheets(shtSheet1 As String, shtSheet2 As String, shtSheet3 As String)
Dim mycell As Range
Dim LastLine As Long, RowNum As Long
Dim nRow As Long
nRow = Sheets(shtSheet3).Cells(Rows.Count, "A").End(xlUp).Row + 1
With Sheets(shtSheet1)
    .Activate
    For Each mycell In .UsedRange
        If Not mycell.Value = Sheets(shtSheet2) _
            .Cells(mycell.Row, mycell.Column).Value Then
            mycell.Interior.Color = vbYellow
            If mycell.Row = RowNum Then GoTo nxt1:
                RowNum = mycell.Row
                Rows(mycell.Row).Copy Sheets(shtSheet3).Cells(nRow, 1)
                nRow = nRow + 1
            End If
nxt1:
    Next
End With

With Sheets(shtSheet2)
    .Activate
    For Each mycell In .UsedRange
        If Not mycell.Value = Sheets(shtSheet1) _
            .Cells(mycell.Row, mycell.Column).Value Then
            mycell.Interior.Color = vbYellow
            If mycell.Row = RowNum Then GoTo nxt2:
                RowNum = mycell.Row
                Rows(mycell.Row).Copy Sheets(shtSheet3).Cells(nRow, 1)
                nRow = nRow + 1
            End If
nxt2:
    Next
End With
End Sub