Try something like:
Sub scratchmacro()
Dim oTbl As Table
Dim oCell As Cell
Dim i As Long
For Each oTbl In ActiveDocument.Tables
For Each oCell In oTbl.Range.Cells
If Len(oCell.Range.Text) < 3 Then
i = oCell.RowIndex
oTbl.Rows(i).Delete
End If
Next
Next