Not a super urgent issue but would be nice to have some guidance.

There are abt 7 med students working rotating evening shifts at my uni's medical library, all of them about 4 semesters higher than me. 4 of them are relevant, all of them know each other:

/#1 I approached last year in November. Was extremely nervous, didnt ask for number but said let's chat again sometime soon. Next day I ran into an ex and decided I only want to win her back and so ignored this girl. Took weeks until I finally chatted her up again, since then we talked like 3-4 times but lost all romantic energy. She's always kinda awkward and seems nervous, at some point I asked for her number and she said "I think no..." in an insecure way. Otherwise very friendly. I still sometimes catch her looking at me but secretly.

/#2 Was eyeing me for a long time, seemed clearly disappointed (almost hurt) when I approached #1 instead of her. One time she saw me with my sister and gave her an angry look, prob thought we're dating. Got a bf a few months ago, think she knows I'm fucking the bf's ex who apparently is her "rival". When I look at her she smiles politely like "sorry I have a bf now" but she obviously enjoys the attention and is still attracted.

/#3 The most attractive one to me personally. For a while last summer she'd give me a flirty little smile every time I entered the library and I'd smile back. We never spoke, one time I chatted up her friend next to her and she just chilled on her phone so I assume she had a bf and wanted to show she's not interested in actually talking. At some point stopped doing the smile thing so now we've been just ignoring each other.

/#4 close friend of #1 and #2. Not interested in this one but she's very friendly and chatted me up in the elevator once. Seems always open to conversation and usually hangs with the other ones. Has a bf.

Atm I'm barely talking to these 4 but there's usually enough opportunity to walk up and talk to them. Any idea what's like, a good plan for a situation like this? Do I pick a specific one I want to go for and escalate with her quickly? Or just do casual conversation for a while and see how each one respond, maybe find out who is actually single and interested before escalating? These women are potential LTR material (smart, driven, socially competent, pretty) plus I see each at least once a week in the library so dont want to ruin things too much.