bicycle_repair_man :
The best thing to do is try them for yourself. How a camera handles is far more important then its technical specification.
One thing to consider when buying an SLR is that you're buying into a system of lenses, flashguns and other accessories. Once you build up a collection, it's very expensive to switch to another brand.
Good point there. I've done it twice and would not recommend that brand jumping be considered lightly.
In my case, I had always shot Minolta so once Minolta came out with a digital SLR I jumped on it since I could continue to use my old lenses ( that 'ole crop factor thing came as a surprise).
Later I wanted to get in to specific types of photography which Minolta (which was now called the Sony A-mount) did not support.
Since I had two good friends who shot Canon so I got a Canon. I sold most of my Minolta/Sony stuff to help pay for it, but you don't get anything close to what you paid for it unless you also bought used. Then I hit a problem which until the Canon 80d plagued every Canon camera to a fairly significant degree for the past 10 years or so. They had significantly less dynamic range and very poor ISO performance. Back in 2009 when I compared a Nikon d90 to the Canon 50d I was shocked at the sensor performance difference. I had assumed that because Canon was used by close to 50% of the Pro's and they sold well to everyone it was just as capable. Then it was explained to me (by one of my Canon using friends) that they stick with Canon because it suits THEIR type of photography plus with many 1000's invested in lenses it was too painful to consider switching. And they could get pretty decent results even in low light so long as they used their $2000 f/1.2 lenses or a tripod (long exposure). Fortunately for me I'd only been with Canon for a year and so only had a few $1000 in lenses and I had a very forgiving wife.
This time when I switched it really stung since I had to pay out several 1000 for new lenses up front to get the same capabilities I'd had with my Canon gear. I've been with the Nikon infrastructure since early 2009. I have no plans to change.
The moral of this story? It is not enough to ask others to compare cameras for you. You have to identify what sorts of photography you want to be involved in. Everyone starts out thinking they'll do everything, and to some degree we all do. But there are almost always one or two types which are simply more important than the others.
Check out some specific camera reviews. Dpreview.com is the best site for impartial reviews. No one serious photographer every completely agrees with everything they say, but that is because a few things are more a matter of opinion than fact. Even so, they are the best overall site by quite a lot.
Check out some sensor capabilities. The defacto standard for this is DXOmark.com A camera system is much more than a list of numbers. My old favorite, the Canon 50d does just terrible there but with that same terrible sensor (mostly with a Canon 500d) I took some of my favorite photos, in part because it had such lovely ergonomics. Meaning the camera seemed to have been made for my hands. More than the Nikon d90 I moved to.
The reason Canon (and Nikon) both continue to sell well is because of their lens selection and the other peripherals such as their flash systems.
Here is the dxomark.com comparison between the camera bodies you have listed.
http/www.dxomark.com/Cameras/Compare/Side-by-side/Canon-EOS-750D-versus-Nikon-D5500-versus-Nikon-D3300___1010_998_928
The Nikon d3300 and d5500 have been editors choice at DPReview. The Canon d750 is not far behind, but it is always rated behind (for example 75% vs 79% review scores compared with the d5500)
Try holding them if you can. Pay close attention to how they feel when you hold them. Good ergonomics is often more important than any other consideration.