Context: I am writing a Systematic Literature Review (SLR) as part of my master's thesis project. After initially screening the titles and abstracts of 338 papers from four selected databases, I identified 138 papers that matched my inclusion criteria. During the full-text review phase, I encountered difficulty obtaining 3 papers out of the 138. I couldn't access these 3 papers because our university wasn't subscribed to these journals, and the Science Hub Mutual Aid community couldn't help either.

Questions: (1) Is this acceptable to reviewers in reputable journals? (2) How should I address this in my SLR? (3) Is it appropriate to email the authors for copies of their papers?