I have a fun math coincidence on my family: every year the birthday for both my parents and I is on the same weekday. All of them is past February, so this is always true.
What's the probability of this happening?
Is it as simple as P(x) = 1/7*1/7*1/7 or do we take anything in consideration to properly calculate the probability? #math #mathhelp #fedihelp #probability #hivemind
It's 1/7*1/7.
What you've proposed would be the probability that all three fall on, say, Monday (or any other specified day).
You could think of it like the first birthday is "unconstrained" - it has a probably of 7/7 of falling on some day off the week. It's only the second and third birthdays that are constrained, to fall on the same DOTW as the first.
(There should be some minor corrections due to there not being an even number of weeks in a year, but that should be small.)
🎂!
tiago 
jz.tusk