Defending-champion Argentina beat El Salvador and Italy managed a tie with Cameroon yesterday to move into the round of 12 in the World Cup soccer championships in Spain.

In a third match, Brazil, already assured of a place in the second round, strolled to victory over New Zealand in Seville.

Italy, Argentina and Brazil will be grouped together in second-round matches scheduled for Barcelona. Brazil has won six and tied two of its last eight meetings with Argentina and its performance so far in this tournament bodes ill for the Argentines' chances.

The other teams that had already advanced are England, Poland, the Soviet Union and Belgium.

Argentina, which began the tournament by losing to Belgium, 1-0, managed to take second place behind the Belgians in Group 3 by defeating the Salvadorans, 2-0, in a bruising battle in Alicante.

The Argentines' opening goal, by captain Daniel Passarella, came in the 23d minute. The team, needing a victory to beat out Hungary for the second-round berth, could not afford to slack off, however, and Daniel Bertoni increased the lead with a left-footed drive after 53 minutes.

The Alicante crowd, overwhelmingly pro-Argentine, displayed a kaleidoscope of banners and flags, many bearing anti-British and anti-American slogans.