There were initially four states slated to vote, but Ohio was called off on Monday night after Gov. Mike DeWine filed suit to block his state's primary, citing concerns that poll workers could be exposed to to the virus.
The St. Patrick's Day primaries are also the first since Joe Biden and Sen. Bernie Sanders faced off in a stripped down, one-on-one debate in Washington, D.C., on Sunday night.
Here's a look at what you need to know:
Which states are voting?
Primaries being held in Arizona, Florida and Illinois.
Extra coronavirus precautions?
In a joint statement on Friday, the secretaries of state of all three states (and Ohio at that time) said they "are working closely with our state health officials to ensure that our poll workers and voters can be confident that voting is safe."
"Further, guidance from voting machine manufacturers on how best to sanitize machines, guidance from CDC on best practices for hand washing, and guidance from our respective state health officials is being provided to every polling location," the statement said.
When do polls close?
Last polls close at 8 p.m. ET in Florida, and at 8 p.m. ET in Illinois and 10:00 p.m. ET in Arizona.
How many delegates at stake?
There are 441 delegates are up for grabs — 219 in Florida, 155 in Illinois and 67 in Arizona.
How are they allocated?
Two types of delegates can be awarded, statewide (or at-large) delegates and district-level delegates.
Candidates must receive at least 15 percent of the vote statewide to receive any statewide delegates and must also meet the 15 percent threshold in a congressional district to receive any district-level delegates. Those who fail to meet 15 percent will be locked out.
How many delegates to win?
To capture the Democratic nomination on first ballot, a candidate must receive support from at least 1,991 of the total 3,979 pledged delegates. According to the NBC News delegate tracker, Biden is currently leading the race with 860, Sanders has 706 and Hawaii Rep. Tulsi Gabbard has two.
When's the next primary?
Georgia was up next, but the secretary of state announced he is pushing back the March 24 primary until May 19 because of coronavirus concerns. That was set to followed by Puerto Rico on March 29, but the Democratic Party there has asked the legislature to postpone the primary to April 26. Four states are scheduled to vote April 4, but one of those, Louisiana, has already postponed its primary until June.