(i hope this is the right thread to post this in?)
I found the theme! Maybe I'm the only one who hasn't realized this until now, but I'll ignore the risk of sounding stupid.
Mayonaka wa Junketsu - MIDNIGHT
Sid to Hakuchuumu - DAYDREAM
Aisaika no Choushoku - Breakfast - MORNING
Also, if you look inside the folder of the single, you see that there are three versions of the cover - each marked in the lower right corner so you can see which song they "belong" to. For you who don't own it, I've taken pictures of my copy.