Take  look at the gtanima.bmp  - This is a single bitmap but contains multiple views of an image. In a sense you could stack up multiple image into a single bitmap file. 

It is not possible to have 3 bitmap files (or gifs)  assigned to the grid cell.

Thanks, but I think I wasn't specific enough.

We have a bitmap strip with a lot of bitmaps and want to show several bitmaps side-by-side in one grid cell.

In one cell bitmap number 1 and 15, in another cell number 3, 27 and 12.

Sorry - the runtime stores the bitmap number as a single value, so setting this multiple times will only keep the last value set (per cell).