The problem with GIF is that you have only 256 colors shared among all the frames(*) and when you add things to a loaded GIF, the colors of the additions are coerced into the colors of the first one. For most purpose, GIF is a "final" format (like, says PDF) and not mean to be further edited (for this you go back to the .XCF (or the .DOC).
If you still have the XCF of the three images, and they are in RGB mode (Image>Mode>RGB) then you can load them over each other. When saving all the colors will be crushed into 256 unique colors, but at least it will be evenly spread out between the three sequence and won't look as bad (but still won't look too good, after all it's only a GIF).
On a side note, animated things and sparkles went out of fashion at the turn of the century
(*) at least for the GIF format as supported in Gimp. Extensions to GIF allow each frame to have its own colormap, but his format isn't supported in Gimp.
If you still have the XCF of the three images, and they are in RGB mode (Image>Mode>RGB) then you can load them over each other. When saving all the colors will be crushed into 256 unique colors, but at least it will be evenly spread out between the three sequence and won't look as bad (but still won't look too good, after all it's only a GIF).
On a side note, animated things and sparkles went out of fashion at the turn of the century
(*) at least for the GIF format as supported in Gimp. Extensions to GIF allow each frame to have its own colormap, but his format isn't supported in Gimp.