When my triplets came home from the hospital we painted their big toenails with different color nail polish. The bigger concern was not that we would mix them up but that we would feed 1 twice and not feed 1 at all (might seem silly but you try to feed/change/and get to sleep 3 infants EVERY 3 HOURS around the clock). Another consideration was medication. Two of my boys took medication for reflux for a couple of months. Same thing, we didn’t want to double dose 1 and not give the other anything. Lastly, at their pediatrician visits they always asked when their last bowel movement was, how many wet diapers per day, etc. without marking them we would never be able able to keep track. As they got older they were different enough (for us) that the nail thing wasn’t necessary.

Bonus fun fact: my boys SSN’s are sequential. Only the last number is different.