In these days of inflation and penny watching, Washington-area residents and businesses flocked this weekend to fetch a three-cent bargain.

Carrying their letters, cards, bills and notes, they rushed to their corner mailboxes and post offices to pay a final tribute to the three-year-old, 15-cent first class postage rate, which was placed at 12:01 a.m. yesterday by an 18-cent rate. The cost of mailing a postcard also increased, from 10 cents to 12 cent.

At the main surburban Maryland post office in Riverdale, postal workers sorted 423,000 pieces of mail on Saturday compared to the normal 235,000. At the city post office, where an estimated 1.5 million pieces of mail are processed on a normal Saturday, the volume was 10 to 20 percent higher. Postal officials at the Northern Virginia center at Merrifield said they also had noted a significant increase, but had no specific numbers.

"My own wife mailed early to save that three cents," said Buddy Hampton, superintendent of mails at Riverdale. "The public was aware of the postal increase and with inflation where it is, it was like catching a sale."

While the postal deluge in Maryland appeared to be evenly split between individuals and businesses rushing to beat the higher-priced postage, the bulk of the increased volume came from big professional mailing firms in the District of Columbia.

"Usually, individuals don't make that extra effort, but the large mailers could lose money," said George Conrad, a spokesman for the main city post office.

Although the first-class rate has changed, mail bearing 15-cent stamps will be delivered at no additional charge through the week postal officials said, because there were no pickups at some boxes between Saturday afternoon and this morning.

The three-cent stamp now is being sold at local post offices to complement its 15-cent cousin. The U.S. Postal Service also has issued a new 18-cent stamp, a commemorative featuring Elizabeth Blackwell, the first woman physician.

And there is the purple-and-white "B" stamp that is something of a transition stamp. It comes with the letter "B" but no price, perhaps to help letter writers forget the amount of the increase. It will be on sale until the U.S. Postal Service can produce enough 18-cent stamps with the price clearly marked on them.

Yesterday's rate boost, which the postal service expects will raise an extra $2.7 billion annually, was the fifth since 1970.