As someone who frequents on tumblr, there are some very problematic people. It's the people who use their disorders or the fact they are part of a minority to undermine others. They claim to be Social Justice Warriors but honestly, things like Down with CIS is not social justice. If someone has a different opinion than them, they will swarm, attack and bully mercilessly. They do not care. Often they all stick together so it's easy to avoid them, but one wrong post can get them onto you.
From my understanding of 4chan (I did do an assignment on it), there is some good and some bad and it depends on what board you are on. 4chan has done some good, they gave birth the lolcat meme as well as the rickroll and Chocolate Rain. They also seem to be vigilantes who are against rape and animal abuse. But there is some rotten eggs which we hear about more, such as the celebrity nudes, child pornography, and other boards which have been mentioned before. But you can only stumble upon these boards if you look for them.
Overall, Tumblr and 4chan, in my opinion, are on par with eachother. I don't know about reddit because I hardly go on there.