I continue for this jam a project I made for Day 3 that didn't reach day 4. Can I update the page of the day 3 project and submit to this jam? Also, do I put the link of my submission in my own submission?

Yes, you can do so if you want, altough it might be good if you leave the day 3 version also available for comparison (you can have multiple downloads on one game page).  👍

Also congrats for being the first submitter! There were a few submissions already but they weren't following the rules so...