Upvote:4
To the best of my knowledge (I lived in Singapore for ~8 years and moderated an expat forum there), Singapore does not publish any official guidelines for repeat visits. However, it's a very small country with limited opportunities for tourism, and as you've discovered, around the three-month mark Immigration's bells start ringing. Once you're flagged, you're usually only given one warning and are likely to be denied entry next time.
You did not say why you're visiting Singapore so often, but the solution is simple at least in theory: get an actual visa. Work visa if you're working, social visit pass if you're visiting friends or family, etc.