Some of these replies are a bit OTT IMO.
Even if you had gone away and taken the 3 yr old, it would be a totally different dynamic with just a 3 yr old rather than with the older ones too.
Same as having a 'staycation' but still having the 3 yr old, as someone else said, he will be at nursery during the day and to bed earlier than the older children, so it would still be a break of sorts, you'd get evenings together.
If this were me and Id planned a break, I too would feel a bit disappointed at not being able to afford to go away, but the older kids going away still would give you space to still have a bit of couply time.
I really don't think he is an Ogre for feeling disappointed.
Won't your older children be looking forward to time with granny anyway? chocolate pancakes for breakfast etc sound way more exciting than just staying home and the usual routine, or worse, being at home and having to tag along on a day trip with your two and their 3 yr old brother, I know it is really hard to please everyone on trips out when the age gap is that big.
What do your older children think about their visit to grannies being cancelled?