First garages are not usually built to be living spaces so you start with an architect who can be up front about the cost for this . I think the idea is good . BTW Love your house so please make sure what you do compliments what you have. I often see bathroorms accessed through closets IMO it forces you to make the closet really pretty and organized

Have you already checked with local rules about converting the three-car garage to a two-car garage? Some places require a certain number of garage spaces per house. Losing a garage space may lower the house value even if local rules allow you to remove it.

You can also consider combining the primary bedroom with the adjacent bedroom. A support beam would need to be added if there's a load bearing wall between them. The third "tandem" parking area in the garage could become a new bedroom. This will all be very expensive.

It’s not really a parking spot that I will be expanding into - there is no way to get a car there . It’s just open storage area