The term 'THE 'S' IN R.S.V.P.' is an acronym for the French phrase 'Repondez s'il vous plaît, ' which translates to 'Please respond.' The three-letter solution for this term is SIL, which stands for the French word 's'il, ' meaning 'IF.' Therefore, the answer to the crossword clue is SIL, which stands for the French word 's'il, ' meaning 'IF' and is part of the phrase 'Repondez s'il vous plaît, ' which translates to 'Please respond.'