When responding to what would generally be considered a "Code 3" call -- in effect, an emergency requiring excesse speed and use of siren and lights -- the first thing the responding officers need to do is keep their emotions under control, as failure to do so can result in...
a bad shooting, use of excessive force in subduing suspects, or sloppy investigative work at the crime scene.
In this particular case, the first officers on the scene would need to physically separate the three witnesses, take statements from each one, then have each one sit down and write everything they witnessed. The physical separation of the witnesses is extremely important in order to prevent them from sharing information, for example, swapping notes on important details like the physical appearance of the suspect. Failure to keep the witnesses isolated from each other would be seized on by defense council during preliminary criminal proceedings as damaging to the prosecution's case.
Once the suspects have been arrested, fingerprinted, photographed, and questioned, they would be placed in a line-up for possible identification by the witnesses. Again, the witnesses have to be isolated so that they do not inadvertently influence each other's perceptions. One-by-one, the witnesses are asked to look at each of the "suspects" in the line-up, which could include undercover police officers, to determine whether one of more of them can identify the suspect.
Questioning of the witnesses, as noted, begins with the request that each of them write down what they saw as soon after the incident as possible to minimize lapses in memory, and to secure a record of their observations before they have had a chance to speak with friends, family, or reporters. Such conversations can taint the credibility of the witnesses, as defense lawyers can raise the possibility that witness testimony was unduly influenced by such interactions with other people.
When questioning witnesses, officers and prosecutors will compare what the witnesses say with what they wrote down in their initial reports. Any discrepencies between the written statements and later testimony can undermine the prosecution's case, so police want to minimize the possibility of such discrepencies emerging.
False identification is always a possibility, especially when witnesses are frightened because they just witnessed a violent act. Having them write down everything they saw, therefore, serves to help them calm down while producing a record attained as soon as possible following the incident in question.