Is the Nikon D3400 classed as a medium format DSLR?
Good heavens no. Not even close.
No, medium format is larger than 35mm/full-frame. It's called medium format because there are various sizes, all of which are larger than 35mm format, but smaller than the smallest of the large formats which ranges in size from 4" x 5", 8" x 10", to 11" x 14" sheets of film. In the film world, medium format is comprised of various sizes such as 6cm x 4.5cm, 6cm x 6cm, 6cm x 7cm, and 6cm x 9cm. All of these different formats use the same 120 or 220 film. Difference between 120 and 220 is just length. You get twice as many shots on 220 film than you do with 120 film. How many shots will depend upon the format you use. The larger the format, the fewer number of frames you'll get. In digital, there a few different sizes of sensors that are described as being medium format. Leica uses a few different sizes, while Hasselblad, Pentax, Fujifilm, and Phase One use the same Sony sensor. All are about 80% larger than a full-frame sensor, which is why they have a crop factor of 0.8. It's less than 1 because they are larger than full-frame which has a crop factor of 1. The D3400 is an APS-H format camera (Nikon calls it DX) which is a wee bit larger than APS-C but significantly smaller than 35mm or FX as Nikon calls it. Here's a link showing the various formats from the smallest up to medium format: http://photoseek.com/2013/compare-digital-camera-sensor-sizes-full-frame-35mm-aps-c-micro-four-thirds-1-inch-type/ If you want to get into medium format, you can actually do it relatively inexpensively, but it's got to be a film camera and not digital. Digital medium film cameras start about $4,000 for the camera without a lens. You can get into medium for literally around $125. Do a search on eBay for a Yashica-mat, Mamiya C330, or a Rolleicord. There are much more expensive models like Hasselblad and Rolleiflex which is the big brother to the Rolleicord and is a much more collectible camera, too.
It is a DX crop sensor DSLR that can take excellent photos if you get the composition and lighting just right. Congratulations.
No. It's a smaller APS-based format which in itself is a little smaller than 35mm/"full frame" formats. Medium format cameras use a much larger image sensor, typically the same size as the 120 roll film cameras they are descended from.
Of course not. It's not even a good cropped-sensor DSLR.