The Met opens its second century with three memorable operas.

The Metropolitan Opera has just begun its second century of existence with one of the better opening-night performances in recent memory - Wagner's ''Lohengrin,'' with a starry cast.

The next two evenings offered more than their expected share of strength as well: Offenbach's ''Les Contes d'Hoffmann'' and Tchaikovsky's ''Eugene Onegin.'' The former made Met history of sorts because of indispositions; the latter offered an unusual bit of casting in the title role.

But first things first. The ''Lohengrin'' was newsworthy because Placido Domingo was singing his first German role in New York; it was made noteworthy because of Eva Marton's blazing Ortrud. In her first try at this sinister role, she strengthened her position at the Met (and with Met audiences) as the most important new star the house has seen and heard in years. She commanded the stage with imperious ease, and at all the important vocal moments the voice soared over the orchestra with cutting power and at times demonic strength. Miss Marton stopped the show for a good minute at the end of her second-act invocation to the gods - an unprecedented happening in Wagner at the Met.

Mr. Domingo was wrestling with a role he had sung only once before, in the late '60s. He brought his special tonal quality to a role that he could become quite good at if he chose to work at it some more. And whereas he did not put the stamp of anything intensely personal on the role, he sang much of it with a tonal beauty rare these days in this repertoire. Histrionically, he at times looked downright uncomfortable with sword, shield, and consistently heroic demeanor.

Anna Tomowa-Sintow has sung Elsa all over Europe. On opening night she acted the role beguilingly well, but it was only in the last act that the voice took on the sheen and thrust one must have to make this part communicate to the fullest. Unfortunately, the remaining roles were taken by singers who were not up to their parts. Brent Ellis's lyric baritone was consistently overpressed by the Herald's music; Franz-Ferdinand Nentwig's Telramund was vocally threadbare and dramatically only partly effective; Aage Haugland sang the role of King Marke in vibratoless shouts that reduced in size as the evening wore on.

In the pit, Met music director James Levine turned on the decibels to an alarming extent, particularly since he cast the opera with lighter-than-usual voices, Miss Marton's thrilling power notwithstanding. Overall, his reading favored broad tempos and thrilling builds to climaxes, peppered with the oddly rushed pacing of several crucial transitional scenes.

August Everding was on hand to restage his '76 production. The Gil Wechsler lighting deletes most of the special effects that once gave the production such a sense of the ethereal. Nevertheless, this ''Lohengrin'' still looks like a grand opera production - and often sounded like one as well.

Offenbach's ''Hoffmann'' is something of a macabre fairy tale, and the current Met production emphasizes that aspect to memorable effect. The first night of this revival, however, threatened to turn into a nightmare. Tenor Neil Shicoff had canceled his performance as Hoffmann early in the day. His cover, William Lewis, had a throat ailment, but agreed to go on. A few minutes into his performance, he was clearly in terrible trouble. Midway through the next scene Mr. Lewis emerged, but the voice emanated from the orchestra pit - Kenneth Riegel had been called in to complete the performance while Mr. Lewis acted and mouthed the words.

It worked out remarkably well, thanks to Mr. Riegel, Lewis's remarkable acting, and the Gibraltar-like unflappability of maestro Julius Rudel. This ''Hoffmann'' had all the drama and beauty one could ask for, put forth with a clarity, even a delicacy, of texture that allowed the singers to be heard clearly, cleanly.

In the ''Onegin,'' conductor Neemi Jarvi repeated his performance of a few seasons back, with even more attention to the lyrical sweep of the score and even more interest in making it a singer's opera. Ileana Cotrubas's acting has rarely been as multifacted as in this Tatyana, and her lyric soprano rang out richly and communicatively all evening long. Vladimir Popov was making his debut as Lenski. Whereas his unsubtle, unpoetic approach to the role tended to jar, the voice is clearly a strong one, with a free-ringing top, and he should be quite useful in this tenor-poor age. Another debuter, Sergei Kopchak, offered a genuine bass of some range, with an easy low register but a rather raucous, even raspy middle and upper range. Anthony Laciura made a particularly fine moment out of Mr. Triquet's little aria.

In the title role, Italian baritone Leo Nucci added an important new facet to his career here. He not only sang the role with all the needed vocal glamour, he looked the part completely. Throughout the evening, one watched Nucci unfold the numerous faces of this haughty character as his Onegin arrogantly trapped himself into a corner that leading to fatal consequences. In the final scene - his only glimmer of redemptive hope - Mr. Nucci, Miss Cotrubas, and Mr. Jarvi rose to a peak of fervor that few previous performances of this opera have attained at this house.