the idol extreme doesn't help you in that regard i think, other than gaining xp. It might that your idols have too low level or you don't have enough memories, but if they are all A1 rank that shouldn't be an issue. What you probably need is better tactics. In difficult festivals i tend to sacrifice the second song and try to get the opponent to waste some memories. Also remember that you can switch out the members between songs. First the memories of the 3rd member are used, then the memories of the second and then the memories of the leader. So you can use the memories of the 2nd and 3rd member without hesitation in each song if you swap out the members after each song.
Another part is the timing of bursts. Who is the opponent in the S3 event and what's their burst skill? I think its Mai Hidaka (Ai's mother), but i'm not sure. If it's Jupiter (though i think they are the S2 rank up opponents) then do everything you must to burst before them and make them counter you. Because if they burst while it isn't a counter, they will strip you of 3 memories and destroy your burst gauge iirc. So it's always better to make them counter you, even if you have to invest 3 memories to do that.