Merge 3 websites into a single website

It would probably be easiest to export the three via Tools -> Export in their Dashboards, then import via Tools -> Import -> WordPress in the Dashboard of the new blog.

Yes,
But how can I manage the changes of category structure

For example for the post1 :
initial website : https://www.site1.com/cat1/post1
after the export/import : https://www.singlesite.com/fr/site1/cat1/post1

Regards

That won’t be possible. I’d recommend making a new category, fr-cat1, for example.

ok, but where create the new category ?
In the new website ?
But Ithe posts and pages of a website in the new site don’t have to merge before I can attribute the good categories for each content of each websites.

To make things easier, create the new category at Posts -> Categories on the old site, then bulk-edit the posts to switch their categories.

Once that’s done, do the export.

Yes, but if I change the structure of the awtual website, I lose the Google SEO.
I want to maintain existing sites in a first time.
And then make 301 redirect

And if I change the author for the old sites.
When I will import the content, the author will different.

Can I change the category struture (adding a prent category for example) for a specific author ?

Ludovic

Yes, but if I change the structure of the awtual website, I lose the Google SEO.
I want to maintain existing sites in a first time.
And then make 301 redirect

Then you’ll have to make the change after the move.

And if I change the author for the old sites.
When I will import the content, the author will different.

I don’t recall saying you should change the author.

Can I change the category struture (adding a prent category for example) for a specific author ?

Sure, but you’d have to do it manually. Create the new parent category, edit the post, switch the category.