I Googled "embroidery designs for triplets" and found lots of images to look at..

You might try the links.

Please Cuties, make sure your links are to sites that follow US and International laws.

How about numerals (there have been many,many cute ones here on cute-hope you saved some)? Put 1 on the first, 2 on the second and 3 on the third.

First off I would put the request in another section perhaps Embroidery so more will see it.
I Googled Triplets designs and all seemed pretty expensive for maybe something used once. I would look at what you have and perhaps combine and make a design up.
Like Michelle does with her Baprons and they always look so special

DT (Don't know which site this is) gave us a design of luggage tags - one says "Buy one" the other says "Get one free." Guess you could make your own labels - "Buy one" and "get two free." Three of any critter, side by side - monkeys would be cute. Check some of the nursery borders given us by Madwoman of Locke Street. Three outfits that say Mom's favorite or Dad's favorite! Three little hearts or three Easter chicks.

How about Kate .Duplikate and Triplikate

On 1st and 3rd shirt: I'm with them, with a arrow pointing in opposite directions for the 2nd shirt have an arrow pointing to each side. Or just say I'm with and an arrow.

Thanks for all the help. Still working on it. The shirts are tiny so can't put too much on them. Love all the ideas. Hugs. Nan W

How about the saying "three peas in a pod" or label #1 #2, #3. One You could say "Threebies"--Just a thought--wished I could come up with more.

I was planned
I wasn't
Me, neither

Set 970218 Toddler Trios from Dakota, if you want to purchase designs. These are sets of 3 little mini designs and are precious on a baby bodysuit and on bibs, whether for 1 baby or triplets. Just another thought. Also, you probably have lots of mini designs from OMA that would be cute in 3's across the outfit.

Disney characters - Huey, Dewey and Louie

In Politics, I'm to the Right, I'm lean to the Left, but what could you say for Dead Center? Might work he he