Dental: Is there a temporary denture (need it) for all 3 furthest back molars for 3 months until bone graft hardens in preparation for 3 implants?

No. I would strongly recommend that you leave the area of the bone graft alone, especially with there being no cosmetic issues. The pressure the temporary denture would put on the graft site during function (transmucosal loading) will compromise the graft's success. The long term goal should be to maximize the success of your long term treatment.
There is.. ... but it would be best not to wear one. The pressure of the denture pushing down on the healing graft can compromise the site. A lot of time & money wasted using a prosthetic that is not needed. Follow your surgeon's advice.