Scanned image

The scans are 4200 x 2800 px, so at A4 print size, you will have approx. 338-359 ppi which will not be pixelated. The quality will greatly depend on the focus of the original shot, physical defects in the film, post-processing (dust and scratch cleaning and sharpening etc). The images may be a bit soft compared to a modern digital camera.

I'd recommend scanning to lossless TIFF, not lossy JPEG.

I personally would use ExifTool to change the resolution metadata as it does not affect the image data.

If you are on a window 10 OS the default scaling is 96 DPI or 100% scaling

Your file may have been scanned at 3000 DPI settings, and have the information embedded in in the file,unfortunately windows will use 96 DPI as a basis for  viewing a file. So your effort is for naught because there is no way to view objects higher then 96 DPI unless you change the scaling ratio from 100% to 5000% so your image on a computer monitor will appear the size of a dot.

So save room on your hard drive and start scanning your images at a max 600 DPI which is overkill unless you intend on printing on 48" x 96" sheets of paper. not 3000 DPI

JMTW

There are two levels of scanner's ppi: actual and digital. The former is what you can do with a telescopic lens, the latter is what you do by interpolation (that is: make up the pixels). I'd be curious as to what kind of scanner you are using because I'd be very curious (and rather dubious) as to their claim. 

I have an epson scanner, a V800 Photo which is low level professional (but certainly not cheap). I use SilverFast scanning software which is professional level software and also not cheap. There's a feature within SilverFast I love: it's when you are "zooming" into the scan it clearly shows you what's optical and what in the transition and what's completley interpolation. About 800 ppi ends the optical, 900 ends the transition and from 1200 – 9600 is made up stuff. 

As far as the use of such images, can't think of one. Magazines typically require a resolution of 300 ppi and high quality priting requires 1200 ppi but that's all for printing. You can use higher ppi but that significantly increases the storage size of the document with no benefits to the quality of the image. For example, most home inkjet printers need only a resolution of 180 ppi for a quality print. 

Now made up stuff may be perfectly fine for your needs and if you wish to use it, please do. The reason that it's showing up with a drop in PPI is simply an issue that all of that data is there, but (I'll bet) your images are ginormous in dimensions. 

Anyhow, to fix this, create an action in Photoshop that sets the resolution you want and save that. Then in Bridge, go to Tools (menu) -> Photoshop -> Image Processor. Set all of the settings you want/need to set but on item #4, look up the Action you just created, and then run this. You can do it for one or hundreds, but I strongly suggest you try it out on a couple of images to make sure that you're getting what you want. Then you can run it on hundreds.

Hope that helps (yeah, I know I gave you lots more info than you asked for, sorry),

File PPI is mostly irrelvant. Total resolution is what matters. If you did set your photos to 3000ppi, they would be the same size as the original negative if printed.