I'm in possession of a gorgeous, squidgy 9 day old baby. His sister is 1 year and 3 months older and his brother is 2 years and 2 months older. Small gaps, as you can see.
When ds1 was born we got a lot of cards and gifts (I obviously kept the cards). When dd was born she got lots, but definitely a few less. Ds2 has had a handful, like 7 or 8. I feel really bad for him 
AIBU to buy some cards and write in them, so in a few years he can look back and not feel left out from the others and overlooked?