The photos show the grown men doing silly things like squeezing into a bathtub, toy chest and cooler together.
"We think it's odd that 2 million people have seen the three of us in a bathtub as fully grown men," MacMillan told ABC News today via email.
Their parents took "tons of pictures" of them as kids, so they had a lot to work with, Macmillan added.
"We took off work and school for a day, so we could take the pictures without our mom knowing," MacMillan said. "We must have taken about 20 pictures of each scene trying to get the perfect facial expressions and poses."
They also searched through half a dozen thrift stores to try to find the best clothes to match what they wore as kids, he said.
And it all paid off.
Mom "loved it," MacMillan said. "She couldn't stop laughing, and then the laughing turned to laugh-crying."
She still texts him every week to tell him she's found something new in the pictures she didn't notice before, he said.
"I suppose I'd say we are momma's boys," MacMillan said. "The youngest is definitely a momma's boy. We have a very close family."