I think you need a structural engineer. The door to the dining room is going to be a function of where it can be put. Is that wall load bearing? After that, figure out where the bathroom will go.

What are you going to do with the garage doors? Will they stay, be walled in with some windows, be replaced with all-glass doors? Is it going to look like this:

Garage Conversion ADU Sacramento, CA · More Info

or this:

My Houzz: Couple's Two-Car Garage Becomes Their Chic New Home · More Info

What space does each of the three families need? Any handicaps?

Assuming each will have cars, where will you park them to make easy access to them?

Much more info needed.

Wouldn't it be less grief to just buy two more homes in the same neighborhood?

First you find out if you can even do this . Usually a garage cannot just be made into living space it has to have proper footings and basically it is a new addition after demolish of the garage . Plumbing a big deal . Are you on septic ? I cannot imagine 3 generations living in one home happily for long . Maybe some more info as to those ages . If elderly people the some at least aging in place ideas need to be considerd. I need a lot more info for sure. I really cannot read the flloor plans very well either . Where are the drawings from the architect ?

General contractor here. Sigrid is right about the structural engineer, and here's the build-side checklist for the garage itself:

Slab: Garage slabs are usually thinner than house floors, sloped toward the door, and sit lower than the main floor. Plan on building the floor up with insulation and subfloor to match the house level.

Garage door opening: Filling it in needs real framing and support at the base, not just studs and siding. It's also where the exterior look gets decided, so plan windows or glass there on purpose.

Bathroom: Putting it on the kitchen-side wall is the right call. Plumbing in a slab means saw-cutting it for drain lines, so shorter runs save real money. A slab also makes a zero-entry shower easier to recess than a raised wood floor does, though it means chipping concrete, so budget for it.

Dining-room opening: Confirm whether that wall is load-bearing before you decide where the door goes.

Code: A bedroom needs a code-size egress window or door, smoke/CO alarms, heating and cooling, and energy-code insulation. Some cities also require replacement parking. Call your building department early, since rules vary by city.

Get drawings and the engineer's input first, then design the layout around what the structure allows.

The bedroom door should not have direct access to the Dining, there should at least be a "vestibule/ante room" or short hallway. The garage slab appears to be lower than the house, so a step/ramp, raising the floor elevation, and if there is conflict with the ceiling height and front wall height because of one of those choices, should be planned for.