One of those three was an 8-year-old boy, and the third person died after being treated for their wounds.
Although media reports have been fluid, between 138 and 144 people were wounded.
Blood filled the streets around the explosions, and inside shops.
Scraps of clothing and flags representing the many nations of those runners participating in the marathon lay shredded on the ground.
So far, 10 amputations have been reported, and other media reports say that they saw the arms and legs of persons lying on the ground. For dramatic photos of the scene, see here.
After the first blast, police and rescue personnel threw the flags to the ground to get to the injured people.
Two people of interest in the crime have been cited by the police department, one of whom is a Saudi national who was admitted for his injuries.
The two explosions occurred within 12 seconds of each other within close proximity to the finish line, about 550 feet apart, and about 4 hours after the race started, at about 2:50 PM. The Mandarin and Lenox Hotels were evacuated after suspicious packages were reported.
Another man of interest was a dark-skinned man wearing a hoody who spoke with race organizers and tried to get into a secure area around the finish line, who was also carrying a knapsack, said police.
President Barack Obama spoke to the nation tonight and noted that the person or persons responsible for this act will be brought to justice.