Lexus has trademarked the RX 350L nameplate in Europe, suggesting that a three-row version of the crossover could be in the works.
Typically, appending an L to a vehicle nameplate means long wheelbase — as an example, the long wheelbase version of the LS flagship adds 4.8-inches between the front and rear wheels. This additional space would make it possible for the RX to accommodate a third row of seats.
(The RX 350L trademark is #015205677, and was filed at the European Office for Harmonization in the Internal Market on March 11, 2016. Thanks Matt!)