BUBBLING BROWN SUGAR -- At the Warner through July 20.; AIN'T MISBEHAVIN' -- At the National through July 27.; 42ND STREET -- At the Kennedy Center's Opera House through July 27.

What we have here in Washington now is a theatrical consumer problem. There are three big restrospective musical shows in town to choose among, and it's hard to say which is the best value. It comes down to being six of one, a half-dozen of another and a sextette of the third.

Two of them, "Bubbling Brown Sugar" and "Ain't Misbehavin'," celebrate Harlem music of this century, in extended nightclub acts that hardly pretend to be musical comedies. Both are packed to the bursting point with exuberant singing, fancy strutting and enough feather and fur boas to stretch from here to 135th Street.

"Ain't Mishavin'," which won Broadway awards as the best musical of 1978, presents the music of Fats Waller, plus a few "songs by others which Fats Waller made hits." Bubbling Brown Sugar," which played Washington in it first year, 1975, has music by, among others, Fats Waller, Duke Ellington, Billy Holiday, Eubie Blake (whose own musical retrospective, "Eubie!," which opened in 1978, will follow at The Warner on July 22) and Cab Calloway, who also stars in this production.

Pick your "Honeysuckle Rose." They're both sweet.

While these cover "The Great Black Way," as "Bubbling Brown Sugar" calls it, the third, the new retrospective "Forty-Second Street" at the Kennedy Center's Opera House, covers the equivalent white way, a.k.a. Broadway. This one your friends in New York haven't seen, and it has by far the most elaborate, expensive and exciting staging. But its score of cute 1930s Hollywood songs doesn't compare with the music from Harlem, and the attempt to string it together with a story from Broadway legend is clumsy.

The story in "Bubbling Brown Sugar" would have been just as embarrassing -- especially the contribution of the white couple serving the function of prudes in porno movies -- if it had been properly developed. But fortunately it's used as the merest pretense for a singing tour of Harlem from 1920 to 1940, or up and down the steps from Bojangles to the A Train. While that cast is "Stompin' at the Savoy," the "Ain't Misbehavin'" cast is "Lounging at the Waldorf," and doing two strong versions of a dozen other songs and a medley, not pausing for nonsense in between.

However, "Bubbling Brown Sugar" has that national treasure, Cab Calloway, now 72, with arms, legs, vocal chords and teeth still all going stronger than anyone's, and it has him doing "Minnie the Moocher," "Nobody" and "Jim, Jam, Jumpin' Jive."

If it's any help, the price range of "Bubbling Brown Sugar" is $8.50 to $14.50, that of "Ain't Misbehaving'" is $9 to $18 and "Forty-Second Street" $9.50 to $24.50.