Tumor in the brain, is 3cm big?

Depends on type: A brain tumor's prognosis depends not just on size, but what type of tumor it is, where it is located, if there are others as well in the brain, and if it can be removed. 3 cm is about the size of a peach pit, but is not necessarily considered large in the brain - it depends on the other factors listed above too.

Depends on type: The type of brain tumor will dictate the seriousness of the 3cm size. If it is a benign well encapsulated tumor like a meningioma, it is tolerable as these grow slowly. If it is a malignant tumor like a gbm, 3 cm may be the epicenter as we know these are infiltrative tumors that expand out and would effectively encompass an area larger than 3 cm. Finally, the location will dictate the seriousness.