Probably you import the collections when you do the 'import from another catalog'.
Collections are saved in the catalog, and when you import this catalog into another one, the Collection goes with it.
But why do you have three catalogs?
There are - very few - reasons for having more than one catalog but the fact that you photograph different subjects is not one of them.
It is generally recommended and best practice to have only one catalog. And the fact that you have to import from one catalog to another shows that the concept of having more than one catalog does not work in your case.
I would recommend that you combine all catalogs into one by using the 'import from another catalog' feature. Then you won't have these problems anymore.