@veronica3 Sorry you're going through this, OP. Dental "insurance" (quotations because most dental plans largely serve as access to negotiated rates rather than actual insurance) has notoriously low annual maximum benefits. In most cases, anywhere between $1,500 and $2,000 per calendar year.
It takes a single crown or root canal to get close to or even exceed the maximum benefit, which means all care moving forward will be out of pocket until the next calendar year. I've maxed my benefits out as early as January for a crown, having to then ride the year out until the following out to resume any meaningful work.
Back to your case - I'm not sure there's "overcharging" going on, rather, balance billing. We can argue all day about the inflated costs of dentistry and the abysmally low coverage thresholds of most dental plans, but we will table that for now. In your case, it seems the dentist's office is billing your dental insurance $X, dental insurance is reimbursing $Y, and the dentist's office is then looking at you to pay the remainder, $Z--likely because for 2023, those three visits / procedures exceeded your plan's maximum annual benefit.
If this is an incorrect assessment, I urge you to upload some redacted images if the bill you're receiving from your dentist along with a redacted copy of the EOB from your insurer to better help illustrate what's going on.
Edit: if your health / dental carrier has a web portal, log in to check any claim history you may have. You can easily tell who has submitted claims to your insurer, when, and for what procedures / services. Highly recommend looking there to confirm the whole "we didn't bill insurance because it wouldn't matter anyway" part of the story.
Edit 2: regarding the dental practice that seems to up to some shenanigans--my expertise falls off a cliff here, but I'd wager there's a relatively simple way to file a complaint with your state's dental board. Definitely keep the pressure up with the office manager--show that they did not adhere to the treatment plan you agreed upon, and that because of that, you're incurring unnecessary costs that otherwise wouldn't have occurred.