What was your procedure in SOGo v2? You made a search by category, selected all results and clicked on the "Write" button? If so, it's the same procedure in v3. Selecting multiple contacts is done by clicking on the avatars. The "Write" action is available in the "More" (vertical dots) menu.

No, my procedure in SOGo v2 is another of the following reason:

By writing a mail from a search in contacts you can only send a mail by using the 'to' field but not the 'bcc' field because the search result goes in the 'to' field only. Sending mails to hundreds of recipients by using the 'to' field means that all recipients can see the email addresses of all others, that's seriously no option in our use case.

This is my procedure in SOGo v2:

• go to the 'Mail' module
• click the 'Write' button
• open the contacts sidebar by click on 'Contacts'
• search for a category
• click under 'Add as...' the Button 'Blind Carbon Copy'

In SOGo v3 there is no contacts sidebar in the mail composer. A solution could be to add one or to make it possible to add the search results in your procedure to 'to', 'cc' or 'bcc' by user choice.