Is this true?
Marginally. Each packet has a slight overhead ( packet size, confirmations, etc ) so sending one larger packet is slightly less data than sending three smaller ones. It's not just the total bandwidth you should be accounting for though. Sending one larger packet is more efficient simply because you don't have to send three times, wait three times, potentially confirm receipt three times, potentially reorder packets three times. Yes, it's definitely better to send your data in moderately sized packets. I'm not sure what would be optimal, but I certainly wouldn't want to be sending four bytes at a time.
I've never done the serializing/deserializing thing, but I did pack my data into a string in KickShot Pool and the networking on that seemed reasonably efficient, although it was not a particularly stressful application in the first place.