Assumptions

From the LA Times:

Exactly how the rampage began remains one of the case's key mysteries. How did a physically unimposing 22-year-old manage to stab three men to death, apparently without drawing anyone's attention? Police are not releasing any details of their findings in the investigation.

There are few assumptions that go with this line of thinking:

First is the assumption that simply because one is male that one is somehow immune to being killed or hurt. The fact of the matter is the vast majority of males are unskilled in the art of self-defense, are not that strong at all and are deathly afraid of physical conflict. Thus the idea that a "physically unimposing' person supposedly could not kill 3 people reveals much about the questioner rather than the perpetrator. It is also this line of thinking that is used to excuse female violence against men, particularly when objects are used by the "physically unimposing" female. This is why I am strenuously against attitudes and policies that excuse female physical aggression against males. While your average female does not have the same strength of a male of similar mass, it means nothing when objects are involved or the target of aggression is unaware of the danger posed. This leads to the likely way that this guy killed his roommates.

Not knowing the time of death for these guys, my first guess would be that he took each of them out overnight by stabbing them while covering their mouths to keep them from crying out and waking the other roommates. As of this writing there has been no comment as to whether there were defensive wounds on Rodgers to indicate that any of the roommates had struggled with him.

The other similar situation would be slitting the throats of each roommate. However; the police have not made such a statement so that's unlikely.

In either case it is highly likely that he got the drop on each of them if he managed to stab each one to death without any defensive wounds.