how hard?

you need to have at least one (and preferably more than one) of 3 things

I put them in the order of importance

1) you know people in those sororities already. not just know of them or anything, being highschool friends/friends from first semester with a girl or a few in house. if you're good friends with someone in a house, they can have a ton of influence on the chapter's decision on you
2) you're hot, just generally very attractive. yeah it's superficial, but with the nature of rush, you only get to know people so well so looks are easy to judge people off of
3) an amazing, standout personality. like not kinda cool or something, like you make a lasting and great impression on everyone you talk to. Since recruitment is so superficial, you reallly have to make an impression for your personality to be remembered
4) GPA/academics, only really important if you are below the gpa cutoff, but if you are, it's super influential

there's several thousand people competing for around 200 spots in the 3 sororities you named, you have to be one of the most appealing 200 out of said thousand

if you are very good looking nothing else matters for most houses

So all girls go through rush. Everyone has top 3 in their rankings. Their pledge class size is around 50. Everyone puts them in.

You do the math.

Know someone who can get you in or be good looking. You only have 5 minute conversations with sisters... nothing your really going to say that blows them out of the water.

Every year, there are gorgeous girls who don’t get bids. It’s all about who you know.