How to combine categories in a search result / archive?

I have several categories that I would like to include in a link, but maintain them as separate categories not bound by a parent-category.

For example, bees, arachnids, and centipedes each are top level categories.

I want to link to (in my menu custom link as well as php displaying posts in a list) all the posts in all these categories?

Can anyone tell me how to do this please?

I’ll take either one (or both):

1) how to link to an archive/search result with posts from all 3,
2) how to use php to display a list of posts from all 3

Thanks