rush questions

First, scroll down to the post titled "Dumb rush question" and you'll find answers to several of your questions.

And the quick answers:
1) No - you never do the choosing - sororities do. The "dumb rush question" goes into more details on this.
2) It's extremely, extremely rare to get a bid from a house you didn't pref. Again, refer to that other post for the details.
3) Don't know about Tri Delt, but my sorority cuts legacies by a certain round of parties so if you get back to my house for pref, that means you're on our bid list. I think most sororities have a similar policy.

1.) you don't choose which sororities you visit after rush. You rank your bottom 3 houses the first night. So if you get cut from one of the houses you liked, one of the house that you ranked as bottom 3 will move up and you will visit them again. This can happen over and over again.
Ex.) I ranked Aphi,Theta,and Adpi as my bottoms 3 the first night. The next day I had gotten cut from 3 houses so Adpi,Theta,and Aphi were on my schedule again. The next time we ranked I put Aphi and Theta on my bottom list again and ended up with them back on my schedule.

This happens because those houses don't really cut girls, which is why most people consider them bottom. Aphi,Adpi, and Theta are kinda like safe houses in a sense that you most likely wont get cut from them. As the week goes on and other houses cut you these will generally always pop back up due to variable quota. It's rare to be cut from them BUT IT IS POSSIBLE (see below)

2.) again, due to variable quota the only houses really offering snap bids are the 3 afore mentioned. I know a girl who was dropped from recruitment completely ended up with a bid from Aphi the morning of bid day. She did not receive her bid card in the grove but was able to attend heir bid day festivities. Theres quite a few girls who this happened too in my dorm. They also offer bids days after bid day.
But no, you will not get a bid from tri delt if you pref DG and KD. You will already be cut from Tri Delts list.

You are highly unlikely to get a snap bid here. That is the only sort of bid that would be offered by any house you did not pref, and the house would have to have not met quota. Not meeting quota would be unheard of for Delta Delta Delta at Ole Miss.