California’s primary system — in which the top two vote-getters, regardless of party, advance to the general election — has seeded plenty of unusual showdowns since it was established a little more than a decade ago.

But never before has the system resulted in a three-way runoff for a congressional seat.

That’s the situation facing three candidates in California’s 16th Congressional District, in the heart of the Silicon Valley, who could vie for the seat during a November runoff. The final count shows two primary candidates in an exact tie for second place.

After nearly a month of counting, both Evan Low, a state lawmaker, and Joe Simitian, a Santa Clara County supervisor, received 30,249 votes, or 16.6 percent each.

They trailed Sam Liccardo, the former mayor of San Jose, who received 38,489 votes, or 21.1 percent.

The three men — all Democrats — were among 11 candidates vying to replace Rep. Anna Eshoo, who opted not to run again after more than three decades in the House.

In California, vote counting takes several weeks because the state relies on mail balloting and gives latitude to voters who wait until Election Day to send their ballots. All county election officials are required to certify their March election results by Thursday.

But the tie for second place is not a certainty: Any campaign or any voter in the district can request a recount, as long as the request is filed within five days of the 31st day after the election — and as long as they pay for it. A full recount by hand would likely cost more than $300,000. That means a recount can be requested as late as next Wednesday.

All three campaigns face a tough decision, said Paul Mitchell, a Democratic political consultant and political data expert. If Mr. Low or Mr. Simitian seek a recount, it could backfire; each could end up paying to knock himself out of the race.

And the campaigns likely don’t have data showing clearly that a two-person or a three-person race in November would be easier to win, Mr. Mitchell said.

Still, he noted that “it only takes one rich, eccentric person” to set the recount wheels in motion. “And they’re on every corner in this district.”

Both Mr. Low’s and Mr. Simitian’s campaigns declined to comment until the results were certified later Thursday. Mr. Liccardo’s campaign also declined to comment on whether he would request a recount.

“We welcome the opportunity to continue to talk to voters about the issues facing our communities, including the high cost of living, utility rates and housing and Sam’s long record of fighting on behalf of residents,” Mr. Liccardo’s campaign said in a statement.