I remember your earlier question about funds parking, your refusal notice showed that it was ultimately a duplicate of UK Visa Refusal: Provenance of funds/parking and now you're ready to apply again. The suspicious transaction has dropped off the back end of the time window, so it will not appear in this application. Your questions...
Do you think, the case officer will have access to previous history
about the refusal and reason for that refusal?
Do you remember the last time you applied for entry clearance you went to a VFS and the attendant asked you to place your hand over a device that looked like a very small photocopier? And then do the same thing with the other hand? And perhaps took a picture of your face with a very special type of camera? For your next application the process will repeat itself.
Those things get digitised and stored on a server. When a duplicate match is found they link everything together and pull up someone's consolidated history. That history will include a high resolution image of your prior refusal notice.
So yes they will know. Although you would never consider misrepresenting yourself, lots of people making UK applications do. So the biometric technology is a valuable asset for them.
Do you see, submitting a Declaration Form about my loan transaction
irrespectively [sic] will help?
Are you asking if submitting an affidavit retrospectively can be helpful? Yes, anything that clarifies and increases transparency can be beneficial. It's not a requirement and you would want to make a very careful front-to-back continuity check covering both applications before including an affidavit. And remember that retrospective submissions can only go in a fresh application; they will never accept something that addresses a previous application unless it's part of a new, fresh application (the rules prohibit this explicitly).
I am planning to submit the bank statement for last 3 months only, so
as to exclude the transaction from August
You may have the wrong end of the stick. What's important here is that it's normal for people to have unusual cash flows from time-to-time. People sell houses, inherit money, get repaid loans, and do lots of things that can create one-off flows. UKVI does not object to unusual cash flows, instead they object to unprovenanced cash flows (big difference) and that was the problem with your previous refusal.
Since they are going to see the whole shebang anyway you can submit a full six months (up to a year) of bank statements and save them the trouble of pulling it up and stitching it together. Moreover, the more statements you submit the more likely they are to think you are being transparent and open.
