Every government is always able to turn totalitarian. That's why voting is important. You don't vote for the totalitarian.
Unfortunately turns out that in practice two of the three don't actually have any power at all when push comes to shove.
Congress does not have an army they can send out to enforce any law they pass, so turns out the president can simply just ignore it all without consequences. What are they going to do?
Exactly, that's the bug. Two of the three branches of government can only write sternly worded opinions on paper. Only one has the brute force to impose their will. So there really is only one branch of government in the US.