<Li> Peter Coventry Smith as Garrett Riggs (season 2), Riggs' half - brother . </Li> <Li> Maggie Lawson as Natalie Flynn (season 3) </Li> <Table> <Tr> <Th_colspan="2"> Season </Th> <Th_colspan="2"> Episodes </Th> <Th_colspan="2"> Originally aired </Th> <Th> Average U.S. viewers (millions) </Th> </Tr> <Tr> <Th> First aired </Th> <Th> Last aired </Th> </Tr> <Tr> <Td> </Td> <Td_colspan="1"> </Td> <Td_colspan="2"> 18 </Td> <Td_colspan="1"> September 21, 2016 (2016 - 09 - 21) </Td> <Td> March 15, 2017 (2017 - 03 - 15) </Td> <Td> 6.54 </Td> </Tr> <Tr> <Td> </Td> <Td_colspan="1"> </Td> <Td_colspan="2"> 22 </Td> <Td_colspan="1"> September 26, 2017 (2017 - 09 - 26) </Td> <Td> May 8, 2018 (2018 - 05 - 08) </Td> <Td> 4.11 </Td> </Tr> <Tr> <Td> </Td> <Td_colspan="1"> </Td> <Td_colspan="2"> 13 </Td> <Td_colspan="1"> September 25, 2018 (2018 - 09 - 25) </Td> <Td> TBA </Td> <Td> TBA </Td> </Tr> </Table> <Tr> <Th_colspan="2"> Season </Th> <Th_colspan="2"> Episodes </Th> <Th_colspan="2"> Originally aired </Th> <Th> Average U.S. viewers (millions) </Th> </Tr>

How many episodes in lethal weapon series 3