This may be KH3 but to simply play only 1, 2 and 3 would be foolish. The “side games” are not side games at all and they all include important parts to the overall plot line of the games. If you have not colected all the games I would highly recommend buying and playing KH 1.5, KH2.5 and KH3D: DDD as this will catch you up on everything.
If you are unable to get your hands on these or don't have consoles you can play them on at the very least watch play throughs of them on youtube.
You will be very lost if you have not played/watched all the games. KH3 starts right where KH3D: DDD leaves off.