okay im alittle skeptical of this but if it is true:

for 1, as soon as you have sex, things will be different, she may become attached to you and want a relationship, so becareful.

2, she just wants sex, just a random hookup, nothing sexual satisfication will come with that.

3, you will have to start with the dating and work your way to a relationship or sex, unless she is loose and sex will be no problem.

i would stick with 1, you already got a friendship with her, and sex will (hopefully) build on that, plus less work for you. however if you want to pursue 3, then leave 1 alone. try not to take both of them 'a bird in the hand is worth 2 in the bush'