I agree with Mr Bipul Kumar.
The notices u/s 143(2) are valid as they have been issued within six months from the end of FY in which return was filed.
As far as your second query is concerned, presently the cases fo scrutiny are selected by CASS (Computer Aided Selection for Scrutiny) software devised by the depatment based on certain parameters decided by CBDT (which is a public document) and also mismatch between information furnished by assessees and AIR filed by various agencies. Further the AO may also select some cases on the basis of information available with him (which includes past records, information received from other agencies and verifiable complaints).
It is normally unusual that the entire family's cases are picked up for scrutiny. Prima facie it appears that during the year there might have been large transactions in properties or huge investments involving all three assessees. In any case, you will come to know the reason once the scrutiny proceedings take forward course.