I assume that your two siblings who have principally dealt with all this have done so following one or more consultations with your Dad's GP and have made sure that they legally can take this step on his behalf. I assume you can, as all of you have a power of attorny, but I am not a lawyer.
My field of expertise is history of religion, and going on that background, I can safely say that you and your siblings have a moral duty to take care of your father, now that he is no longer fully competent to make his own decisions.
Most of us feel slightly guilty when facing your dilemma, but really there is no need. You are all doing your best for your father in a very difficult situation, and managing it without falling out with each other in the process. Few siblings do so well!
Someone now needs to tell your father of these plans and agreements you have put in place.
If at all possible, I would suggest that all of you( that is his children ) should be present, delegating the actual telling to one person.
This precaution ensures that you all know exactly what was said and whether your father took it in at all, and if so, how he reacted.
You need all to be prepared for him either refusing point- blank to conform, or at the other extreme forgetting immediately what has been said.
However, the step is necessary for his well-being and for you and your siblings' peace of mind, so the matter will have to be broached.
Depending on how much your father still takes in and remembers of what is said to him, you may find it best to leave the actual announcement until as near the time of his move as possible.
Please do all remember that as dementia increases, it becomes more and more dangerous for a dementia patient to live alone, and that your father both now and later will be far better off being cared for professionally.
If you leave it much longer, you might end in the situation two brothers I have known since we all were children ended up in. Their mother was found wandering in the roadway outside her house unable to find her way back home, at four o' clock on a winter morning when the temperature was well below freezing, clad only in nightie and slippers!