While the news of these three deaths is tragic and suspicious, I doubt that there was actually fowl played involved, especially not from 'foreign intelligence agencies'.
For one thing, I just do not see why anyone would care enough to try, and even if there were hostile powers so inclined, there are more important projects they could be more interested in disrupting, things like the J20, WS10A, WS15 engines etc.
While project directors are critically important people and their deaths would no doubt set back the projects they are in charge of, no-one is irreplaceable, and there can be no question that the J16 or J31 programmes might collapse because of these deaths. Others will step up and the programme will survive and come to fruition.
Just look at the Y8 AWACS programme, a plane filled with experts and engineers, who are probably more important to the programme because they are actually responsible to doing the research and development rather than mainly managing, was lost, and the programme was still ultimately a success.
In addition, if there really was some conspiracy, it would be unlikely that anyone with the resources and access to assassinate top Chinese officials would be so stupid as to mount several assassinations in such a short period of time that even us - interested laymen, would become suspicious. I have no doubt that the relevant Chinese intelligence and counter-intelligence agencies are taking a another long hard look at these deaths again now.
This is not like Iran's nuclear programme where there is a ticking clock and where the target is effectively powerless to strike back. I cannot see any imminent time sensitive issues that would cause someone to throw caution to the wind and mount multiple assassinations in a matter of weeks rather than months or years; for another, the risk factor involved in green lighting such a mission is massive.
As already mentioned, it is almost a certain that these deaths are being scrutinised closely again, increasing the chances of detection of any fowl play, in addition, if China ever managed to get evidence of assassinations, China has all sorts of options to strike back hard, be it politically, diplomatically, economically or even militarily.