All three of these Campaign types are Automated Campaigns from Google Ads that use Machine Learning to optimize the Campaigns. But they have slight differences in their nuances. First the mechanicals. Smart Campaigns need seven days only, whereas Discover and Performance Max need a 2-week and 6-week learning periods respectively. Secondly, objectives, Discovery is best suited when you want to raise brand awareness and reach and are only served on Google feeds. Performance Max is aimed at generating conversions (maximum conversions or maximum conversion value). Smart Campaigns allow users to choose their goals from a larger list of objectives like getting customer calls, visits to your location, website sales or leads, or views on YouTube and partner websites.