In a surprising move, the North American Soccer League voted yesterday to increase the required number of North Americans on the field during games from two to three for the 1980 season.

Many in the league had expected the measure to be voted down, since a number of teams had difficulty coming up with two North Americans last season. But league owners an directors, meeting here this week, voted in favor of the switch in what was reportedly a close vote.

The league's master plan for the 1980's calls for an increase in North Americans on the field every two years until the number reaches six. Each increase must be approved by league officials, however.

At the same time, the league voted to increase the number of games next season from 30 to 32, but voted down a proposal to increase the number of intradivisional games from two to four. Instead, the league will use the same format as last year, the two extra games coming with additional interconference play.

The length of the season also was expanded by two weeks. It will start March 29 and end with the Soccer Bowl Sept. 20.

The site of the Soccer Bowl will be decided today. Mayor Marion Barry will make the presentation on behalf of Washington and RFK Stadium. Washington is considered a strong favorite to get the game.

As expected, the owners voted not to make any rule changes for next season. The 35-yard line will remain, as will overtime, the shootout and last year's often confusing playoff system.

The league did make one slight change in the bonus-point scoring system As in the past, a team will get one standings point for each goal scored in a game up to three points. But no longer will goals scored in overtimes or in shootouts be counted as bonus points. The move is designed to cut down the advantage a team has had in winning in overtime or in a shootout.