Q: Why have I received three notifications at different dates for the unchanged status "Ready for Decision"?

Once the manuscript has been submitted, the journal editor carries out a preliminary assessment. If the manuscript clears this assessment, it is sent for peer review. Based on the reviewers' comments, the editor takes the final decision. In your situation, it seems from the three notification messages that you received, that your manuscript has passed the editorial assessment and the manuscript has already undergone peer review.

“Ready for decision” means that the peer review is complete and the editor should send you the decision soon. It is likely that the editor is assessing the peer review comments and will arrive at a decision regarding your manuscript soon. The manuscript status in the online submission system also corroborates to this. I'm not sure why you are receiving these automated emails, but it looks like the system sends these emails whenever someone accesses your manuscript at the backend. It is possible that the editor is going through your manuscript to arrive at a decision.

I would suggest that you be patient and wait for the editorial decision. You should receive the decision soon. However, if you do not receive the decision within 2-3 weeks, you can send an email inquiry to the journal editor. 

Related reading:

• Peer review process and editorial decision making at journals
• The peer review process: challenges and progress