There are two main ways to identify the suspect in this case, one is more convenient and time efficient for law enforcement (and tends to happen more often) and one is more rock solid in court in terms of admissibility.

Either using three squad cars or taking the witnesses over one at a time (but in some way keeping them separate when they view the suspect), I would give the three witnesses the chance to see the person being held.  Some cars have tinted windows so the suspect cannot see who is identifying them.  This would tell me whether to make a preliminary arrest and would justify a search of the suspect or their car/property.  The other, more traditional way involves putting the suspect in a lineup and bringing the witnesses in one at a time at the station.  While at the liquor store, I would also analyze the video of the crime (all such stores now have video security, for the most part) and see if the height and build matched the suspect.  A photo lineup could also be used where photos of five people are taken and the witnesses are asked to identify who they think the robber was.