And in each case, it appears no existing law would have prevented the gunmen from obtaining their firearms, and all three legally purchased the weapons. Still, the mass carnage has renewed debate about whether it's too easy to buy a gun in the United States, especially semiautomatic rifles like those used in the shootings.
Here's a look at the weapons and how various laws could have potentially impacted the ability to purchase the guns: GILROY, CALIFORNIA The 19-year-old gunman who killed three people at a festival in Gilroy used what authorities called an AK-47 variant, but they haven't described in any detail what kind of features it had. He purchased it from a gun shop in Nevada, where he was a resident at the time. Authorities have said it was legally obtained in the state, where anyone 18 and older can buy a rifle.
However, an AK-47 variant cannot be legally purchased in California, which bans a variety of long guns it classifies as assault weapons. And under a California law that went into effect Jan. 1, residents younger than 21 are barred from buying rifles or shotguns unless they are in the military or law enforcement.
EL PASO The gunman who authorities say killed 22 people in an attack on at a Walmart store was armed with an AK-style firearm and an extended capacity magazine. The AK-47 was developed in what was then the Soviet Union in the years after World War II. AK, which stands for Avtomat Kalashnikova, was designed by Mikhail Kalashnikov, a Russian general and arms designer. It tends to have a wooden stock and a curved, long detachable magazine.
DAYTON The gunman used an AR-platform rifle with a 100-round drum magazine. AR stands for Armalite Rifle, its original manufacturer, but the style of weapon is now made by several gun companies and is considered the most popular modern sporting rifle in the United States. They were banned for a decade under the federal Assault Weapons ban but existing owners were allowed to keep the firearms in their collections; since that ban expired in 2004, the weapons have only gained in popularity and notoriety, with gun-control advocates decrying their use in mass shootings.
More than 8 million AR-style guns have been estimated to have been sold since they were first introduced to the public in the 1960s, and about half of them are owned by current or former members of the military or law enforcement, according to the National Shooting Sports Foundation.
While the vast majority of mass shootings are carried out with handguns, some of the most high-profile and deadliest mass shootings in recent years have involved with AR-platform long guns, most notably the gunmen in Las Vegas, Newtown and Parkland.
Drum magazines are legal in most states and can be purchased for about $100. As the name suggests, rounds are stored in a cylindrical magazine allowing dozens of bullets to be compactly stored. While it allows for a large number of rounds to be fed into the firearm's barrel, drum magazines make the firearm heavier to carry.
BACKGROUND CHECKS All of the gunmen appeared to have undergone a background check before purchasing the firearm. Under federal law, people convicted of domestic violence and other serious crimes, drug users or those who are the subject of a restraining order for harassment or stalking are among those who are barred from possessing a firearm.
Despite the numerous warning signs surrounding three young men, nothing in their past would have been spotted in a background check and kept them from buying or possessing a firearm. Authorities say the gunman in Dayton purchased the gun online and that the firearm was delivered to a federally licensed dealer in Ohio; such sales are subject to background checks.
He reportedly had been suspended from high school years earlier after making alleged threats. But he apparently was never charged criminally, either in juvenile or adult court, and it also doesn't appear that he was ever involuntarily committed, two avenues for denying someone the right to purchase a firearm.
Gun control advocates have long sought nationwide universal background checks, which require a background check on virtually every firearms sale, including private sales between family members and at gun shows.
Currently, 11 states — California, Colorado, Connecticut, Delaware, New Jersey, New Mexico, New York, Oregon, Rhode Island, Vermont, and Washington, and the District of Columbia — require background checks for all sales and transfers of all firearms. The state of Nevada will join the list in January.
RED FLAG LAWS Several states have red flag laws that allow authorities to confiscate the weapons of unstable people. Gun-control advocates say such laws can save lives since often there are warning signs before a mass shooting. Gun-rights advocates argue that such laws can be used maliciously by people with a grudge and also violate due process.
Such laws had no impact on the ability of the three gunmen to get their weapons. Texas and Ohio don't have red flag laws, and it doesn't appear that the gunman in California, which has such a law, had exhibited behavior that could have led to a confiscation of his guns.
Red flag laws are on the books in 17 states and the District of Columbia.