JAZZ Monty Alexander, Ray Brown, Herb Ellis: ''Triple Treat.'' Alexander, piano; Brown, bass; Ellis, guitar. Concord Jazz CJ-193. Monty Alexander is one of the most important newer voices in jazz piano. Jamaican-born, he infuses his style, which bears more than traces of Oscar Peterson and Bill Evans, with blues and Carribean rhythms. On this album, his second with Ellis and Brown, the mood is refreshingly simple, with a platter of delightful tunes ranging from his own calypso ''Fungi Mama'' to Hoagy Carmichael's charmingly innocent and too-seldom-played ''Small Fry.'' Few pianists around can construct a line as lyrically and cleanly as Alexander - and with a touch of wit, too. With the dependable and swinging Brown and Ellis joining forces with him, nobody will notice that there's no drummer - a most enjoyable set.