So when the Saotome's arrived and it was confirmed that the girl was indeed Ranma, Soun fainted in disbelief How could a miss like this have happened?
After Soun was back awake, and Genma Saotome told soun the story of how it all started when they Journeyed to a legendary training grouns and had fallen into the cursed springs at Jusenkyo China.
The Girls Still Unaware of the story or being capable of believing it Later find out as Akane Did that Ranma is a Man, who is cursed with the fact that whenever he is covered in cold water that Ranma changes into a beautiful young girl who fell into the Cursed Spring and died. and ranma changes back to the person he is when he is covered in hot water...
So the 3 Daughters of the Tendo Family early on are perplexed with the fact that ranma is a man after they initially met ranma the woman and liked her and felt they slightly knew her, and that one of them is being set up by their father to marry him..