IMO curbless is good, but is pointless unless the bathroom door and other doors are wide enough for a wheelchair. There are designers and other professionals who are certified in aging in place/ADA. Look at the big picture, including grips, etc.

3' is pretty narrow to keep water in. more importantly, you need the vertical space to drop the shower floor below the rest of the bathroom floor (or to raise the rest of the bathroom floor so that shower is lower), which can be hard in a renovation (where the current floor is likely right on top of the joists). might be worth it, might not.

you can do a very low curb + lots of grab bars that should get you through most of aging-in-place issues (with a couple of stools added in). wheelchairs are, of course, another thing entirely, but not necessarily something that everybody needs at any point.

My friend did a rounded speed bump shape as a curb in his shower. I don't know how that would work with the general public.

I have a curbless with a rain glass panel for privacy. Opening is enough to get wheel chair in, To have proper draining of water we made sure the grading was done correctly.

You can use a 3/4" thick tile threshold as the divider between the bath floor and the shower floor. That will stick up only about 3/8" above the other tile surfaces and the glass door should be centered on the threshold. Your space is pretty small for a curbless shower but it is not unheard of either. I would make sure that the contractor and tile setter are well versed in how to create a curbless shower and the various products available. An untrained/inexperienced individual can ruin it quickly. Ask for references on past projects that include curbless showers and contact the references. If you can go see one then that is even better.

Attached is a link to a Houzz search on small, curbless showers.

https://www.houzz.com/photos/bathroom/query/small-curbless-showers/nqrw

We have 2 curb less showers and love them. There is no basement under them so the floor slants down into the drain. I really love them. One is wheelchair accessible and one isn't. They are way less of a fall hazard

Here they are called zero entry. Had mine for19 years. Recently rebuilt shower. Here are the issues. The new schluter base we used does not have the slope that our old base does and the water will pool a bit before it goes into drain. If your sewage backs up for any reason or your drain clogs your shower has no containment so into the bathroom it will go.

IF your contractor cannot construct a curbless without leaking, then believe him. His leaks. Hire someone else. :-) The only downside is the added space needed and the expense to an already expensive project. How much additional expense depends on how much structural work needs to happen to ensure that you get the right slope with the trench drain at the back wall of the unit in the 48" dimension. That will give you the 2" drop between the entry to the shower and the drain that will keep water inside it.

My dad's shower wasn't that large 40"x40" with a bench in it. He had PArkinson's and the last 5 years of his life ( when I was there) I got in the shower with him and gave him shower...It had a curb. It worked fine. we even were thinking of these trick bench/seat deals where a person can slide into a bathtub..I think maybe the geriatric aspect of curbless showers is overplayed. I'm remodeling 2 baths with showers myself and am also wondering about curbless showers. Bathroom remodel and the costs are already soaring so maybe not! Those frameless shower doors (3/8" glass) ain't cheap!! Undermount sinks..crikey!! Where is the money tree!!

Although this is an old thread, i wanted to emphasize the point made above…in a water backup the water level quickly exceeds the slope of the shower floor and escapes the waterproof stall. It will quickly start running to lower points.
We had this happen…We have wall mounted toilets, so our lowest point in a backup was walk in (zero entry) shower stalls. We had damage to baseboards, drywall, furnishings…..
No one expects a backup, but when lightening strikes it is awful…especially when you learn that homeowner insurance usually has low coverage caps for this type of water damage.
I like my zero entry showers, but knowing what I now know I might have gone with a traditional lip on shower stall. An overflow can still happen, but at least you have more time to react and get a clog cleared before water escapes.