I'm not a big fan of Nikon's D3xxx series of cameras. Really for just one reason which is that these models (e.g. D3100, D3200, D3400, ect...) don't have internal auto-focusing motors. This means that you'll be limited to buying specific lenses that have AF motors built in. Nikon has about 5 different F mounts, some with and some without internal AF motors. This reduces the types of lenses you can use and it really makes buying lenses (especially used ones) very confusing. Many people have purchased used lenses on eBay just to find out that it doesn't auto focus on their D3xxx Nikon. So if you absolutely have to get a Nikon for whatever reason, go with a D5xxx or D7xxx series. There's nothing wrong with buying a used camera. In fact, as a beginner who doesn't know much about photography, it's actually a smart way of getting into photography. As you learn more about photography, you'll eventually upgrade. In the mean time, it doesn't make much sense to spend a lot on a new camera when you could get a fully capable used model and spend the money you save on better glass. Bodies come and go, but good glass will always be good glass.
The major weakness with Nikon is their horrible video capabilities. Nikon never built camcorders like Canon, Sony and Panasonic did back in the 80s and 90s. As a result, they don't have the know-how to build a DSLR with decent video capabilities. On the other hand, because of their 30-40 years of experience in video, Sony and Panasonic have the absolute best video performance of any digital camera. Canon is tops among DSLRs, but Sony and Panasonic are superior when it comes to video.
Nikon Strengths: Very popular and a result, there's a ton of used gear at really cheap prices.
Canon: Take a look at any of their DSLRs within your price range. Canon is by far the most popular DSLR especially among beginners. Canon puts the AF motor in their lenses so any APS-C format body will accept any auto-focusing lens for or by Canon. Entry-level models like the T5, T6 and T7 are only sold with a kit lens. While this is a fine lens for the beginner, if you want to get better glass, you'll need to go with a higher-end model like an 80D which is sold body only, or, with a better kit lens.
Pentax: Take a look at the Pentax K-70 which is probably the best APS-C model in its price range. Pentax doesn't have the sales that Nikon or Canon have, therefor as a result, Pentax has to put more into their DSLRs to attract buyers. This is to your advantage as Pentax is the only DSLR to offer in-body image stabilization (IBIS). IBIS means that the sensor moves to stabilize the image, and as a result, any lens you use will be stabilized up to about 4-5 stops. With Canon and Nikon DSLRs, you have to spend more on stabilized lenses. Down side is that not all lenses are stabilized.
Pentax is also weather and dust proof. You don't get that kind of seals with Canon or Nikon until start spending more than about $1,500 on the body only.
Pentax uses pixel shift technology - the common problem with sensor that use the Bayer pattern is that there aren't an equal number of red, blue and green pixels. This results in lower sharpness and color quality. Pentax solves this problem by taking multiple shots and then blending them together resulting in a significant boost in sharpness and color quality that can't be achieved with any Canon or Nikon with a comparable lens.
For the beginner and advanced user who does landscapes and/or low-light hand-held photography, Pentax offers features that really provide an advantage over the competition.
Sony has done a fantastic job with their mirrorless cameras. Prior to the Alpha 6500, most mirrorless cameras were not capable of focusing on moving subjects as DSLRs. Now that's all changed with the A6500, A7 and A9 series of cameras. Go on line and do a search for the advantages of mirrorless vs a DSLR as there are many. Mainly due to the electronic viewfinder (EVF) used in mirrorless cameras. With an EVF you see the exposure in the viewfinder allowing you to instantly know if the image is going to be over or under exposed. With DSLRs, you're looking through an optical viewfinder (OVF) which cannot show you a preview of the exposure. There are many other advantages and I encourage you to do some more homework.
In terms of learning photography, there's nothing about a mirrorless that will hinder your studies. They just as capable as a DSLR with just as good image quality.