It is possible for a twin fetus to disappear from the sac without visible bleeding. This phenomenon is known as vanishing twin syndrome. In some cases, the vanished twin may be absorbed by the mother's body or reabsorbed into the placenta. It is important to note that even if one of the fetuses disappears, it does not necessarily mean that there are complications with the remaining pregnancy.
In your case, since your ultrasound at 9 weeks showed one sac with a heartbeat and another empty sac, it is likely that you are now carrying twins instead of triplets. The disappearance of one fetus could be attributed to vanishing twin syndrome.
It is understandable that you would like further clarification on your current situation. However, if your doctor has advised against having another ultrasound at this stage, it may be due to medical reasons or because they believe it will not provide any additional information or change the course of your pregnancy.
If you have concerns about your pregnancy or would like more information about vanishing twin syndrome, it is recommended to discuss them with your healthcare provider. They can provide personalized guidance based on your specific circumstances and address any questions or worries you may have during this time.