I am all for refusing to take the third patient in ICU as it's unsafe.
I don't know the way things work in your hospital, but if you refuse this patient, where else in the hospital will the patient go? Another ICU? Will the patient be held in the ER? And if so, will this cause problems for them (where I live, this might mean that incoming patients will have to be diverted from one hospital's ER to another). Does that mean that an unstable, high acuity patient will be kept on the floor, where the staffing may be 6, 7 or 8 to one?
Bottom line is that you can
refuse the patient as long as your manager is willing to stand her
ground and support your decision. It's an issue you need to discuss with her.