What is the life expectancy for an 83 year old man with diagnoses of alzheimer's, parkinson's, and lewey bodies dementia?

Depends. Depending on how severe the above conditions are, and what other medical problems he has, it may be anywhere from 6 weeks to 5+ years. For your loved ones, particularly if they are mentally incapacitated, you would want to address end of life issues such as would they want to be maintained on a respirator, live in a nursing home etc. A social worker can help you with that.
Hard to be specific. The short answer is that an 83 year old man with serious medical conditions has a limited life expectancy. Being more specific would require knowledge of many different specific details related to this person. This persons physician would be in a much better position to give a realistic answer. Signs that someone is nearing death due to dementia include weight loss and decline in walking.