I'm so sorry you had a rotten experience. I feel your pain!
I've heard of movie theatres having special matinee showings that are specifically designated for parents and little kids (toddler/preschool age). People go to these showings knowing and accepting the fact that there will be kids running around. I've also heard of late-evening showings being restricted to a minimum age, but it's much less common. (Actually, neither setup is particularly common.)
It's expecting a lot to expect a 2-3yo to sit still for 1.5-2 hours, but you're absolutely right that mom should have taken the kid outside when he started acting up. Just a couple of minutes outside might have been enough for him to settle down. Or not, in which case you stay outside a little longer. The best thing to do with a little kid is leave him with a babysitter (which can get expensive) or with family (if you're lucky enough to have someone nearby). Or wait for the movie to come out on DVD.
The older kids' behavior was inexcusable. Kids need to be taught from a young age that certain behavior is unacceptable in public. And part of that is the parents setting a good example themselves. You watch, that 2yo is going to be just like his brother in 10 years.
On a related note, someone mentioned a 1yo at an R-rated movie... I went to see the South Park movie, which is the only time I've actually seen a theatre card for an R-rated movie. You wouldn't believe the number of parents I saw with their 3- and 4yos. I'm sorry, but I would
not want my (hypothetical future) 3yo seeing Big Gay Al in his altogether!
Parents should prescreen the R-rated movies they take their kids to see.