Whether or not it's totally beating the odds that there indeed would be 3 transgenders in the same class of your brother, I don't think the openness about 'gender-fluidity' etc. is a bad thing.
You might have never known gays until the early 90's as well, simply because it wasn't a subject openly discussed. You might have never met vegans until the media hyped the diet. Hell, if it wasn't for the public figure Dalai Lama you could've never heard of Buddhism!
It's only by opening up a subject, people get comfortable and accepting with it.
If there would be separate bathrooms for that 0,01% (accounting for 75.000.000 people, deserving the same basic human rights as for men and women) there would be a more understanding culture towards transgenders. Like you said: you were never in contact with tranzies. And I doubt you do now.
But like the God you claim to make people says :
RE: Why Being Transgender Shouldnt Be Accepted