If Joni Mitchell had two funny, younger sisters, they'd be Kate and Anna McGarrigle. All three sing with the same fragile emotion, the same deceptively simple but timeless lyrics and the same strains of Canadian folk music.

In fact, "Pronto Monto," their third and newest album, has more in common with Mitchell's appealing early work than Mitchell's own current, unfocused material. As Mitchell often does, Kate McGarrigle implies the powerful emotions in seemingly casual relationships.

Mitchell crystallized a uniquely female perspective on common situations in many early songs, and Anna McGarrigle's Bundle of Sorrow, Bundle of Joy" is a similar landmark, a rare acknowledgement of the contradictory feelings surrounding motherhood. Anna doesn't cheat on either side of the contradiction, but maintains the paradox as she rides the folk-rock momentum.

Both the McGarrigle sisters fill their lyrics with precisely painted landscapes and small gestures, though neither has pulled off the spectacular images of Mitchell at her best. But both have a clownish sense of humor that Mitchell usually refrained from. Kate's "NaC1" is the story of the romance between sodium and chlorine molecules in the ocean. Her "Side of Fries" proves that women can have as much fun sizing up men as physical specimens as vice versa, and Anna's Dead Weight" has this classic put-down line: "You're a dead weight/And I can't wait/To see the back of you."

Having grown up in Montreal, the McGarrigles emphasize the French traditions of Canadian folk music much more than Mitchell (who's from Alberta). The suspended, delicate French musical pasages are given momentum by American folk and show music influences. The sisters play a range of instruments themselves and are ably supported by old Montreal friends, producer/guitarist David Nichtern and singer Bonnie Raitt's beloved sidekick, Freebo.

But most impressive are their vocals. Each sister sings lead on her own compositions and supports the other with harmonies like flickering candle shadows. Having grown up to gether on the same piano bench, their vocal rapport is so close the harmonies often seem as inevitable as shadows.

Eric Kaz, another writer who also sings, is better off when he lets other, greater voices take over his material. Kaz's "Love Has No Pride" and I'm Blowin' Away" are large, impressive sails, but his voice simply doesn't fill them as well as Bonnie Raitt (who sings his "Cry Like a Rainstorm") does in her bluesy belt.

After several overproduced and undersung solo album, Kaz formed American Flyer, a quartet with Craig Fuller of the Pure Prairie League, Doug Yule of the Velvet Underground and Steve Katz of Blood, Sweat & Tears. Unfortunately, the group's two albums were not nearly as impressive as the members' resumes.

Now Kaz has formed a duo with Fuller and released "Craig Fuller/Eric Kaz." But they don't really operate as a duo. It's as if each one recorded half a solo album and then joined the other one late. They only appear on tow ongs together.

Fuller had a hit with "Amie" when he was the leader of Pure Prairie League. Both his singing and songwriting here are in that vein: pleasant but undistinguishable from the countless laid-back country-rockers cluttering up Los Angeles.

Kaz is a more considerable talent. He's capable of an extravagantly romantic lyric: "Prayin's for the sun/To bake the poisons from my bones./ Oh no, these tired wings/Have turned to stone."

But his real talent is creating ballad melodies. The above song, "The Ways of a Woman," is typical in the way the verses keep raising your interest without satisfying it. By the chorus, one is leaning forward in empathy for the singer's sadness. When the emphatic melodic clincher comes, one gives into the fates of love as surely as the singer does.

The album is supported by the best of L.A.'s musicians-four from Jackson Browne's last album and three from Ronstadt's last record. But Kaz sabotages his own songs with a tendency toward overkill arrangements. Just as he reaches the payoff of "The Ways of a Women," he brings in a sentimental string quartet to helabor the point. "Til You Come Back" is ruined by a grandiosity that cancels out the humility of the song's plea.

Recently there have been a lot of great voices hungering for good songs. Rondstadt, Raitt, Emmy Lou Harris, Maria Muldaur, Carlene Carter and Judy Collins all have excellent voices but write little of their own material.

Many of the decade's finest songwriters are primarily known for their songs sung by these six singers. Some - such as Kaz-are better off letting great voices take over their material. Others-including the McGarrigles- benefit from the exposure. But often. songwriters like the McGarrigles suffer when their own songs, enriched by fulfilled intentions, are obscured by less personal interpretations. And unfortunately, when their own personal albums do appear, they often are lost in the shadows cast by the spotlight on their more famous clients.