Thousands of blacks will get a chance this week to sit in first-hand judgment on the three remaining major candidates for the presidency, all of whom are scheduled to speak at the annual convention of the National Association for the Advancement of Colored People, which opens here Sunday.

The NAACP, the nation's oldest and largest civil rights organization, says that it expects some 16,000 of its members to attend the five-day convention, which has as its theme: "Vote! Be Heard."Scheduled to address sessions of the convention are Vice President George Bush, the Republican presidential candidate, on Tuesday morning; Jesse L. Jackson, the Democratic runner-up who remains officially in the race, on Tuesday evening; and Massachusetts Gov. Michael S. Dukakis, the Democrats' presumed nominee, on Wednesday morning.

For each of them, the NAACP audience will be the sounding board for a kind of dress rehearsal for the campaigns to come after their party conventions.

If the audience is at all representative of the national black vote, it will consist mainly of those who supported Jackson through the primaries and who have gone over to Dukakis only because the Democratic race for the presidential nomination is, for all practical purposes, over. Thus, polls conducted nationally in June by the Gallup Organization showed Dukakis garnering the support of 73 percent to 75 percent of the blacks who were polled.

Nevertheless, the NAACP audience will be waiting to see how much enthusiasm - or at the least, political friendship - Jackson will show toward Dukakis. And if the matter hasn't been decided beforehand, they will be looking for any sign that Jackson may still be pressing to be Dukakis' running mate on the Democratic ticket.

On the basis of recent polls, Bush will face an uphill battle with this audience. A Gallup poll taken on June 10-12 showed Bush receiving the support of 18 percent of the blacks who were polled, but a subsequent Gallup poll, taken on June 24-26, showed Bush's black support dropping to 10 percent of those polled - a fall of 8 percentage points in two weeks.

The reason for the sharp decline was unclear, but the two polls contained a margin for error of 3 percent and 4 percent, and the proportion of blacks who chose neither Dukakis nor Bush was 9 percent in the June 10-12 poll and 15 percent in the later one.

But even if Dukakis may no longer have the matter of his choice of a running mate to deal with - or avoid - as he faces the NAACP conventioneers, he will be under heavy pressure to set forth positions on civil rights and other black concerns. In an apparent trial run last week, Dukakis met in Boston on Wednesday with about 20 civil rights leaders. After the meeting, the spokesman for the group, NAACP executive director, the Rev. Benjamin L. Hooks, said that the session had been "cordial and productive."