Last november, we had a group in front of us in the line for Ralph and Vanellope at DHS ... there were 16 kids total, and one adult had her arms full of homemade autograph books. Kids all had their pictures taken as a group, as smaller groups (with multiple permutations including pictures with parents (on and off the picture).
Was it long ? Yes it was. Was is acceptable ? Yes it was. It was because the kids waited in line like anyone else. What difference does it make if your 3 kids did the line 3 as 3 separate "parties" ? They waited patiently in line, they should be entitled to having their picture taken with the character.
Can you imagine your 3 kids waiting in line for a ride, and only let one of them ride because letting all 3 go on the ride would mean to take an additional row, thus delaying another family ?
Anyway that's how I see it, even if I only have one kid to take care of.
Of course, if there is a long line, then one picture for the 3 of them would be a nice gesture towards the wait time of other guests behind you, especially if you know that you might encounter that character at some other time.
Now there is a way to speed up things a bit.
Don't send your kids one at a time for the autographs. Send them all to get the autographs, then have them take a picture together, and then tell the photographer that you'd like a picture with each kid. It's really easy and fast to have them take turns with the character, for a nice hug and a picture.
It's especially efficient when you meet several characters at the same time. For example Goofy and Pluto or Dug and Russell at Animal Kingdom. Or when you meet Chip and Dale in MK, or Phineas and Ferb in DHS. Each character can sign one autograph book, you effectively do it faster for the people behind you.
But your kids deserve their autographs and picture, and in the end, you'll realise that it's faster than you imagine (WDW makes it very easy for the meet&geets, DLP is definitely not in the same league here)