I opened 3 girls today.

First was when I was going to classes, she seemed to be in a hurry to work or something. We had to walk on the same bridge to take the bus so I approached from her side and said "I got a minute and I need your opinion because youre a woman" she looked at me with a face that indicated surprise but also a smile. "You see, I was wondering if woman forget a long term relationship so easily" Then she started talking to me but since we were on the bridge I could barely hear. After that she went to her bus but just before taking it she turned to me but I just stood there and well she took the bus.

Second one was at the institute, she was going downstairs after classes, I used the same opener and she was like "Are you really talking to me?" (she seemed like a girl who was kinda pretentious) so I just said the same shit and she started answering that "obviously no! most girls dont forget that fast" after that I didnt continue and just went my way.

Third one was in the bus to home, she sitted next to me and I asked her inmediatly "do you think long term relationships are easy to forget?" and she looked at me like "WTF?!" and said "Im sorry" and went to another seat.

Now that I think in retrospective, I feel that I could open and gain their attention, at least with the first one but I just didnt know how to go on from there. I need some advice

That opener is weak.

It's a yes/no answer, first of all... and it's pretty generic and transparent.

You don't say what you talked about after the opener - which is really the important part... You have to transition into something relevant or funny... capture her attention.

You have to understand when to use opinion openers and when not to. The girl on the bus sat next to you and you asked her opinion on something without even saying hi. If she sits next to you, find something you like about her and comment. "I like your style", "Love the scarf, you pull it off nicely". Just something. And keep the conversation going from there. Also, an opinion opener is better if you can make it seem like your not about to quiz the person. So do it like this:

"Hey can I get your female opinion on something real quick? I gotta meet up with some friends in a few minutes but what do you think of..."

I'd also get a new question to ask if you're going to ask for opinions. It just doesn't seem interesting.

Most important, when that girl turned to you before getting on the bus, ask for her number.