Lessons learned on keeping communication with pre-school positive and flowing. You could let them know about specific signs to look out for, you could keep him home if it's edgy.
Additionally, it was only 3 hrs ... And as others have mentioned, other parents may feel (or not be able to pick DC up at moment's notice) that 3 hrs tolerable for whatever it was.
You felt 3 hrs ok if teething issue. Only when - after the fact & more information - did you find out it was more than teething. Your pre-school carers aren't doctors nor are they omniscient. Neither are you (please give yourself a break, as well. This will be long haul.).
Suggest you have more detailed plan in place with current and future schools/care givers so that DS is looked after, you have peace of mind, and second-guessing after the fact doesn't become routine.