We’ve had rolling suitcases from Pottery Barn for our kids for several years.
https://www.potterybarnkids.com/search/results.html?words=Luggage

I think those are challenging for kids that age to carry. Especially for a long trip, they would be heavy and stuffed.

Personally, I would each have them be responsible for their carry on book bag which would be full of their toys, stuffed animals or things to do on the plane. I would just take one suitcase for all the kids and use packing cubes to organize everything - one for all swim, one for all pajamas, one for toiletries, one for 5 year old clothes, one for 3 clothes, one for infant clothes, etc.

I would look into a wheeled suitcase + a small bookbag if you really want them to each lug their own things.

No, OP, NOOOOOOO! That is a TERRIBLE choice. It will be way too heavy for them to carry. That strap is going to bite into their little shoulders SO hard. They will either refuse to carry or drag it along the ground.

Get them each a wheeled suitcase. Something like this: https://www.potterybarnkids.com/search/results.html?words=batman%20luggage&cm_sp=HeaderLinks-_-OnsiteSearch-_-MainSite&cm_type=OnsiteSearch&typeahead=legacy+batman%20luggage+

First, I think it is a great idea for each person to have their own bag and learn to be responsible for it. I think I was six or seven when my parents started having us pack (after a parental check of what we were planning to pack) and carry our own bags.

Having grown up with the round Lands' End duffels, they are a bear to pack. It is round! Stacks of clothes don't go in. That said, the duffel I had from age 7 to my teens was smaller than that and fit clothes and minimal toiletries (toothbrush and paste, hair brush) just fine. Not sure I ever needed a second pair of shoes at that age.

With our DD, she received a carryon sized rolling bag and matching backpack when she was 3 or 4. I think it was from One Step Ahead or a similar site. She graduated from that to an adult size bag around 9 or 10.

Another option is just to get different colored packing cubes for each family member. Then you can share a suitcase but make it a little easier to find things. My SIL uses this system.

Size-appropriate backpacks for the 3 and 5 yo. Anything rolling is a nightmare and just extra work for you. A duffel or shoulder bag is harder for them to carry.

Going on a long trip this summer with my 5 and 3 year olds and infant. I want an individual piece of luggage for each kid so that I don't have to fish through a big suitcase with all 3 kids' stuff in it every day. Obviously I'd carry the infants. Do you think this would work for the 5 and 3 year olds? https://www.landsend.com/products/medium-seagoing-duffel/id_320247?sku_0=::3K4

Waht do you all use for your kids? I want to teach them responsibility for their stuff as well as keep stuff organized. Obviously we'll have to help them sometimes carry it/pack it.