Hey there, sorry for the slow reply, can I ask a couple of questions which may help us to work out what might be going on?
Just looking at that last nap - on days when she goes down at 2pm, do you still wake her at 3:15, or is that when she wakes naturally? I just wondered whether she may actually be getting a bit OT and having trouble settling for that reason, especially on days when she had a shorter second nap? Or have you tried a 3h A time after a 1.5h second nap and if so, was that definitely UT? How long ago did she drop the third CN?
When did she start solids? It sounds like she's on quite a lot for just 26w, even if she's taking minimal amounts, even those tiny quantities can cause discomfort for immature digestive systems. Did you see any correlation between the disrupted nights and starting solids?
When she was most recently sleeping better, what was her routine then, how long ago was it, and how much sleep did she get on average per 24h?
What are her NWs like? Is she screaming, fussing, fidgeting or chatting/playing?