Are you merging them for any specific purpose (printing, publication, etc...)?
The result would be enormous... may even lead to crashing the program when you want to save.
The only was to make it less demanding would be decrease the resolution, but doing so really sucks on maps.
Merging three big items would probably require resolution that would render the labels unreadable.
Plus, uploading to the guild, if you plan to do so, may not work out with a large file. But I encourage you to try it.
Best of luck