Intercepting Sprout Forms "onSaveEntry" Event In Craft 3 Plugin
I've updated the Sprout docs with examples of Craft 3 events and hooks. 

In your case, the Form Entry Element extends the craft\base\Element class and also supports the same after the base element save event: EVENT_AFTER_SAVE. In your primary plugin class init method:

use barrelstrength\sproutforms\elements\Entry;
use yii\base\Event;
use Craft;

public function init()
{
    parent::init();

    Event::on(Entry::class, Entry::EVENT_AFTER_SAVE, function(Event $event) {
        if (Craft::$app-&gt;request-&gt;isSiteRequest)
        {
            // A Form Entry Element has been saved from a Front-end Form Submission
            Craft::dd($event-&gt;sender);
        }

        if (Craft::$app-&gt;request-&gt;isCpRequest)
        {
            // A Form Entry Element has been updated from the Control Panel

            Craft::dd($event-&gt;sender);
        }
    });
}


You can access the specific Form Entry Element being submitted via the $event-&gt;sender property.