In order to send your post to several social media networks, you can use Multiposting feature.
Regardless the way you start - either from the Editor or Calendar or Post Ideas section - you finish at pre-posting stage where you are to select a social page. Feel free to tick more than one network on the top, like it is pictured bellow.
Besides, you can choose several social platforms, when using Repost feature. But be advised that Multiposting can't be applied to a scheduled post, but to a published one only.
IMPORTANT: Please mind that visuals do not resize automatically when the post is sent to various networks, because their post sizes are optimized differently. So pictures might not look the same as on the preview step.