The nation will learn sometime next week if it is in for another three years of labor peace from its biggest postal union, or if it must prepare to cope with a nationwide mail strike with Army troops sorting letters and patrons picking up their own mail at the post office.

Officials of the American Postal Workers Union, in convention here, report that nearly half their 260,000 members already have returned mail ballots indicating whether they want to accept or reject a proposed new contract. If the members of this group - and of the National Association of Letter Carriers and the Mail Handlers Union - approve the contracts, postal employes will be guaranteed layoff protection for the next three years. They also will get a combination of pay raises and cost-of-living adjustments that will bring the salary of the average postal clerk and letter carrier to more than $18,000 a year.

If members reject the contract - the counting of the mail ballots begins after Aug. 23 - it will send union negotiators back to the bargaining table. They would have a 15-day time table either to come up with a new, improved contract, subject to membership ratification, or to call a strike of the Postal Service's half million workers if the offer is deemed unacceptable.

This month's conventions of both the Letter Carriers Union and the APWU voted overwhelmingly to reject the contract, repudiating most of their national leaders who signed it. But it is still up to the rank-and-file workers, who would go payless on the picket line and risk jail, fines, and loss of jobs, to decide whether the contract is so bad it justifies an illegal strike.

APWU president Emmett Andrews, who has been accused by some members of failing to bargain hard enough, has already informed national officers of this union that they - like their members - will do without pay checks if he is forced to call a strike.

The situation is complicated by the fact that three AFL-CIO unions involved have agreed to act in unison. This presumably meas that if members of two unions vote to accept the contract, and one rejects it, calling for a strike, all union members would honor the picket line. But insiders believe rank-and-file members of the unions probably will vote the same way. The problem is that nobody knows whether they will accept or reject the negotiated contract proposal.

Three years ago, only 130,000 members of this union voted in the contract ratification process. Today - with five days still to vote and mail the ballots - more than 122,000 APWU members already have voted. Their ballots are being kept under lock and key until the union counts them sometime after the Aug. 23 deadline for their receipt.

The big, early turnout definitely means something, a union official noted. "But I'm not sure what," he added. "It could mean that the anti-contract people turned out to vote no. Or it could mean that people who want to accept it got in early to block any possibility of a strike. We just don't know."

Most of the delegates and union leaders I have talked with this week believe the contract will be accepted by members. However, even the most optimistic feel it will be a close vote.

Militant delegates here continue to criticize most of their national officers, but generally in moderation. This is in sharp contrast to an outburst Monday that delayed the convention for nearly two hours as demonstrators stopped all business while they demanded that their president resign. The militants have won most of what they came here for - a denunciation of President Carter, knuckle rapping for their own leaders, and the convention's nonbinding rejection of the contract.

The delegates voted today to trim future raises for their 90 national officers by altering the complicated system which links their pay to percentage increases given regular members. The exact dollar effect is unsure but union sources say it will mean raises of about $400 a year for top officers, instead of the $1,200 they might have received under the old system.

The 3,000 union representatives here voted against a resolution that would have made at least nine of their 23 top Washington-based officers ineligible for reelection because they are postal retirees. It would have barred the incumbent president and several other national officers up for reelection, for running.

But they did approve a resolution that will keep anyone who retires after Jan. 1, 1979, from running for state, local or national office of the union. The move split the convention generally along old-young, active-retired lines. But it carried. The change will not affect any of the incumbents, or anyone who retires before next January.

Union-watchers say acceptance or rejection of the contract will be decided in a show of mail ballot strength between big city workers, led by New York, who oppose the wage package, against employes in smaller towns and the far West where postal wages and jobs are considered among the best in the community.

People who know the membership breakdown of this union say the match-up is about 50-50, meaning that the side which mails the fastest and the most will decide whether everybody's mail goes through.