Possibly the easiest method here providing the table schema is setup correctly is to write your insert as a 'select' statement so in this case possibly along the lines of

SELECT <column 1>,<column 2>,<column 3> .......

FROM Table1 left outer join Table2

WHERE <Enter Clause Required>

once you have the data correct add and 'Insert into Table3' clause above the select to insert into Table 3. Just make sure the columns in your select match the columns in your table.

Its noit bullet proof but should put you on the right track.