It is always a pleasure to welcome back Combo Fiasco.  The trio's harmonies, combined with their musical dexterity -- the two boys swap duties at the piano like footballers using the interchange bench -- make their cabaret show an occasion of joyous music-making that is positively contagious.     Tony McGill and Shaun Murphy formed the combo with Sharon Millerchip when they were all singing in The Phantom of the Opera and made an immediate impression with their collective charm, style, good humour and an abundance of that elusive but essential component, chemistry.  They worked so closely as a unit that they seemed indivisible.     With Millerchip's departure, the delicate balance was upset, but Charmaine Clements, also from the ranks of musical theatre, assumed the distaff role without skipping a beat.  Hers is not the same gamine appeal of Millerchip, but she is sexy, funny and talented, and the new chemistry with her is just as potent.     In fact, Clements contributes the highlight of this new show -- Diva, a terrific revue number spoofing television "stars" promoted beyond their vocal capabilities to play lead roles in musicals.     She sings exquisititely off pitch and runs the gamut of mechanical movements in a wickedly hilarious parody of bad acting.     It is one of several new itesm in their ever-changing act.  Others include a biting burlesque of trashy novels, Bestsellers; a knockout setting of Windmills of your Mind; and some slick topical lyrics to I'm Stuck on You that bring big laughs with references to John Howard and Senator Bill Heffernan and the inadvisability of inviting Wayne Carey to a barbecue.     But old favourites score just as well -- the perennial Fats Waller medley and other swing-era and gospel songs, their subversive comic numbers Sensitive New Age Guys and Wouldn't It be Nice to be a Lesbian, McGill's energetic Masochism Tango, Murphy's finely sung Time to Say Goodbye and Clements' gorgeous Someone to Watch Over Me.     They rejoice in close-harmony singing and do it spendidly in a well-balanced show distinguished by taste, inventive musical arrangements (in which they are assisted by Colin de Sario, bass), and a boundless sense of fun.