This sounds like something happened to corrupt the Android operating system, causing your phone to just get stuck in a boot loop. (With no functional OS, all your phone can do is boot up into either its Recovery mode or its Download mode.)
Just for reference, to intentionally boot into one or the other:
Recovery mode -- start your J3 while simultaneously holding down the Volume Up, the Home, and the Power buttons
Download mode -- start your J3 while simultaneously holding down the Volume Down, the Home, and the Power buttons.
To restore the OS to your phone, you need to flash a stock, Samsung ROM. Flashing a smartphone ROM is similar in principle to using a restore disc/media on a PC to fix a broken OS. Go here to download the necessary ROM you'll need:
https://www.sammobile.com/firmwares/search/galaxy j3/
or here:
https://updato.com/firmware-archive-select-model/?q=GALAXY+J3&exact=1&r=&v=&rpp=100
Both sites claim to verify the ROMs they provide as being legitimate, Samsung stock ROMs.
Can't stress this point enough but be sure to select the ROM that matches both your phone model (i.e. SM-J320P) and your carrier. Do not substitute or try other ROMs just as an experiment. ROMs are specific their matching phone models. If you use the wrong one and you're lucky you'll just soft-brick your phone and have to re-flash using the correct ROM. If you're not lucky you might hard-brick your phone.
The chosen download pages of both include instructions on the flashing process. An example being:
https://updato.com/how-to/how-to-install-an-official-samsung-stock-firmware-using-odin/
Follow the directions carefully, if you use the Odin utility correctly, there's a good chance your data won't be wiped (a given being if your data is already gone than flashing a ROM won't restore it). A successful flashing of a stock Samsung ROM should only affect the system partitions, leaving the data partition as is.
