The three candidates, Said Sadi, Abd Allah Djab Allah and Ali Benflis, in a statement hours before the keenly contested polls closed on Thursday also warned of "serious consequences" if results failed to confirm the trend.
"By the light of information collected by all the offices, it emerges that the general trend is towards a two-round vote," the three candidates said in a statement.
The three had earlier accused Boutaflika of seeking to rig the results for claiming victory with 53 to 55% of the votes.
"By the light of information collected by all the offices, it emerges that the general trend is towards a two-round vote"
Statement from three presidential candidates
The statement did not say which of the six candidates – who also included the incumbent Abd al-Aziz Boutaflika - would advance to a second round.
The three also warned that "any announcement that contradicts this trend will be considered as an attempt to put before the nation a fait accompli without regard for the serious consequences."
Benflis is the leading challenger to Boutaflika. Sadi is the head of the secularist Rally for Culture and Democracy and Djab Allah is an Islamist who heads the National Reform Movement.
Recently liberalised electoral laws allowed candidates to follow votes from polling stations to the final tallying centre. This was also the first election in which the powerful military has pledged to stay away.
About 46% of the country's 18 million voters cast their votes during eight hours of polling.