Yea, you could define all bitmaps and store their handles in an array. Something like this:

bmap t1 = <t1.bmp>;
bmap t2 = <t2.bmp>;
bmap t3 = <t3.bmp>;
var bitmap_array[3];
starter store_bmaps()
{
bitmap_array[0] = handle(t1);
bitmap_array[1] = handle(t2);
bitmap_array[2] = handle(t3);
}

then in the function use:
function random_panel()
{
var i;
i = int(random(3))+1;
bmap_ptr = ptr_for_handle(bitmap_array);
transform_l01_normal.bmap = bmap_ptr;
}