These are fairly recent songs (though Hachimitsu is from like 1995), but not written specifically for this series. I love the fact that they are using different outside music each episode. They’re all guitar heavy male vocal songs and I really like all three. I wrote the romanji wrong for the song titles, but here are there corrected names (with my limited knowledge in the field):
Spitz – Hachimitsu (ハチミツ) (Episode 01 Insert Song)
Suga Shikao – Hachigatsu no Serenade (8月のセレナーデ) (Episode 02 Insert Song)
Suga Shikao – Tsuki to Naifu (sp?) (月とナイフ) (Episode 03 Insert Song)
Links to their official sites:
Spitz
Suga Shikao