Ah a subject close to my heart.........my trio will be 16 in January and it as happened several times that they have been sent a card between them, even Christmas. Personally it upsets me more than them. They are three individuals, they have to share so much anyway being triplets that they should have their own personal card..........everybody else does! It happens with gifts too. I know its expensive but as said you can always pick up cards cheaply these days, we have The Card Factory shop in town and you can get 8 cards of pretty good quality for £1!
I think aged 1 I would just send them the one card. In future years, I would send separate ones, but at this age are they really going to notice or care? 3 gifts is obviously important, but I don't think even the parents would mind at 1. I would be relieved I didn't have to find enough shelf space for a zillion cards!