A very sad start to the year.
The trouble with a PA28-140 is that it can catch the unwary - I owned one for several years and have instructed on the type for many years.
Although it has 4 seats, it is by no means a 4-seater. More sensibly a 2+2.
It would be almost certainly over-weight with 3 on board and full fuel (pictures on the Web show it parked outside - so maybe it still was and with full fuel to reduce the chance of water contamination). It will DEFINATELY be over-weight (and possibly uncontrollable in certain situations) with 3 on board if operating in the Utility category (that is, carrying out certain manoeuvres, such as spinning). There was another sad accident some years ago when two very experienced airline pilots and instructors crashed in the sea after spinning a -140 off the Fylde Coast. The aircraft was over-weight for the exercise and CG out of limits.
As an example, I have flown the -140 with 3 on board and 3/4 tanks and 4 on board with (just) 1/2 tanks. Any more with average weight males on board and you are outside the envelope. It is probably one of the trickiest aircraft in common use in this respect and is not tolerant to over-loading. I checked the weight and balance carefully if I went flying with more than 2 on board (which was quite often as I used it for flying family and friends and hours building at the time).
No speculation about the cause of the crash, just sadness that a enjoyable day out should end in tragedy.
KR
FOK