Everyone said it already, racism is normally treated as the worst of the three because it has such a long storied and violent history, and yes the same can be said for all the other three, and I'm not saying it's the right view. But in those three, persons will react differently depending on which one it is, and racism causes the worst reaction, that is true.
Sure, you can say that people shouldn't always bring up the slavery aspect of it all the time, but you can't deny it is one of the biggest examples of racism, so that's why it's the first thought people arrive at when they hear that word, and it wasn't only slavery, people used to be killed just for being of a different skin color and race. But while you say the phrase "we were slaves" shouldn't be used as an excuse, it's not like the phrase "It's because I'm a woman" hasn't been used from time to time. And though sexism is and will always be apart of our lives, you can't deny that things have been made better for women, much better. In fact, you brought up the ways women were treated in the past, isn't that the same as people falling back on the "slaves" excuse? Just saying.
The fact of the matter is that it's in our mentality to distrust or dislike things that are different to ourselves, everyone's a bit racist or sexist, not in the fact that they will actively detest or try to hurt people different that them but in the fact that they will label those who are different with certain stereotypes whether consciously or subconsciously. And though, some will be able to think above those stereotypes, there will always be that small inkling in the back of your head that someone isn't trustworthy or someone is suspicious because of a difference in looks, behavior or sex. It is just automatic for example that because someone is a woman, they're supposed to be weaker than men, or that because your employer is a man, they're fighting against you preventing a woman's success even though there might just be another person who's just better than you at the job.
Now I'm not saying that there aren't those sexist, racist individuals or homophobic individuals who hold others down but the fact of the matter is people will sometimes attribute individuals succeeding in fields they have failed in to them being treated differently or coming from a different background : "He's asian, no wonder he beats me in maths" or, "If I was male, I'm sure they'd have given me that promotion". It happens, it doesn't mean that you're racist or sexist, it's just you falling subconsciously into stereotypes. I've been in NY, and been followed in shops by security guards because I'm a big, tall teenage black guy, and I'm not saying those guards are racist, they're just following the stereotype, which is not their fault but the fault of the black teens who in the past may have stolen, and maybe caused trouble in their store thus their mind automatically associates anyone who fits that stereotype to be a thief or cause trouble, and they treat them accordingly.
Alot of the things that racism is screamed out for IMO are just misunderstandings, who's to say someone of the same skin colour would not have been treated the same way?
Anway, I barely touched on homophobia. Probably because I suspect I'm a bit homophobic myself, not that I detest them or anything like that, I'm a bit indifferent to them, just people who choose to live their life differently to mine, more power to them. However, I do get annoyed sometimes at homosexuals who deliberately draw attention to themselves, causing commotions and what not all to accuse individuals of being homophobic, when really they were just reacting to you being an ass.
But I guess there are those in every sexuality. What I don't agree with is how homosexuals have been treated, more importantly the fact that they are killed in some places. People say that God didn't like homosexuals, even so, you think God would like you taking the choice of life or death into your own hands? Think God likes murderers?
To (finally) conclude ( I hate typing so much), those 3 have been terrible acts by human beings to human beings, some of which is still taking place right now, but some are treated worse than others due to simply the fact that they have had more violent pasts and vivid occurrences than the others. Even if laws are being passed to reduce and prevent all three, there is NO denying that they are still present.