That is actually pretty normal for stage 3 tins, lah, because at that age kids are eating more solids so the milk is more of a supplement than the main nutrition. My girl's stage 3 tin looked noticeably less full than her stage 2 ones too, and I had the same shock the first time. It is worth checking the gram weight on the label versus the earlier stages, as the serving size and recommended daily intake do change, so the tin tends to last a similar number of days even if it looks less packed.