Tremolo with more than two notes

[Helge Kruse]

Since the documentation says that only two notes tremolos I think this is a 
feature request and not a bug.

I need to write a piece with a tremolo like fast repetition. But since
there are three notes in this repetition, the
  \repeat tremolo n { x y }
doesn't work. How can I achieve such notation?

Minimal example:

\score {
   \new Staff {
    \time 3/4
    \repeat tremolo 2 { b8. c' }
    \repeat tremolo 3 { b8. c' d }
  }
}