It’s not possible to send campaign to multiple segments at a time. However, you can easily create a new segment, which combines the leads of multiple segments with the help of Standard Segment.
First, you need to create a Standard Segment. Click on Segments and then Create new.
Select Standard segment and then enter a segment name. Also, select Segments from the dropdown list.
Be sure to select Leads match: All, and Segments: any of the selected segments. Then select the multiple segments. After that click Create segment.
Once the segment is created, it will show the lead count and subscriber count of the combined multiple segments.
When you click on the created segment, it will consist all the leads of the combined multiple segments.
Now create an email campaign and select the newly created combined segment.