Three Sons-in-law

An old woman had 3 daughters. One day she decided to test her three Sons-in-law. 
One a fine day, she was walking along a lake shore with the first son-in-law. Purposefully, she fell down in the lake and started yelling for help. 
The first son-in-law jumped into the water and dragged her out into the shore. The next day he found a brand new E Class Mercedes in his door steps with the wordings: "Thank you! Your Mother-in-law who loves you very much!!" 
Another day she went out with her second son-in-law. Purposefully, she fell down in the lake and started yelling for help. 
The second son-in-law too jumped into the water and dragged her out into the shore. The next day he found a brand new E Class Mercedes in his door steps with the wordings: "Thank you! Your Mother-in-law who loves you very much!!" 
The third time she was walking with the third son-in-law and she repeated the same. But that guy got scared and ran away without offering any help to her. The poor old lady who wanted to test her sons-in-law drowned and died. 
The next day the third son-in-law was surprised to see a new brand new Rolls-Royce waiting at his doorsteps with the following wordings: "Thank you very much! Your Father-in-law!"