Two cars collided and three people died. What does he have to do?

Traffic accidents should be referred to the experts in that
field and eyewitnesses who saw what happened; this brief description is not
enough. 

If the experts rule that one of the two parties was negligent
or aggressive, such as if he drove in the opposite direction or he cut off
the other driver, or he was careless in changing tires which led to his car
turning over and hitting the other car, then he is the negligible party and
is liable for the deaths of others and the destruction of property. There
may have been negligence on both sides, in which case each party is liable
for damage caused to the other, whether that be damage caused to people or
property, to the extent to which they were negligent. 

The Islamic Fiqh Council issued a statement on traffic
accidents in which it says: 

Accidents involving vehicles are subject to the Islamic
shar’i rulings on criminal acts, even though they are usually caused by
errors. The driver is responsible for damage caused to the other person,
whether that is physical harm suffered by him or damage to his property.
After deciding who is at fault and the extent of damage caused, he is not to
be absolved of blame except in the following cases: 

1 – If the accident happened as the result of something
beyond his control and he could not avoid it.

2 – If the damage is caused primarily by the one to whom the
damage is done.

3 – If the accident resulted from negligence or aggression on
the part of another, and the other is to be deemed responsible. 

And they said: If the driver and the injured party both
caused the accident, then each of them is liable for physical damage or
damage to property caused to the other. 

Shaykh Ibn Baaz (may Allaah have mercy on him) was asked:
There was an accident in which two cars crashed into one another. In one car
there were two people, one of whom died. According to the police, the driver
of the first car was 30% responsible, and the driver of the other car was
70% responsible. With regard to the kafaarah (expiation), should the driver
of the first car fast for two consecutive months or for a time proportionate
to the degree to which he is to blame, as is the case with diyah (blood
money)? 

He replied: If two or
more people are jointly responsible for killing someone by mistake, then
each of them has to offer expiation independently, because expiations cannot
be split between individuals, as the scholars have stated, 

Fataawa Islamiyyah, 3/360 

The Shaykh was also asked: 

When my father was driving a car, he crashed into another car
and the driver of the other car died, may Allaah have mercy on him. The
police confirmed that the accident was entirely the fault of the deceased
and the family of the deceased agreed to forgo the diyah (blood money), may
Allaah reward them with good. My question now is: Does my father have to
offer the expiation of fasting for two consecutive months or not? 

He replied: If the situation is as you describe then your
father does not have to offer any expiation because he is not to blame and
he cannot be described as a killer. 

Fataawa Islamiyyah, 3/356. 

The point of quoting
these fatwas is so that you may know the importance of following the rulings
of the experts and eyewitnesses and finding out the details so that it may
be determined who is to blame and to what degree. 

The fact that the accident was not caused by a deliberate
action does not mean that the blood money or expiation can be waived. 

And Allaah knows
best.