Ouch, something like a modem crash error is a nasty one. Try powering off your phone and then restarting it, and since your J3 Pro has a removable battery you might want to remove it, wait several minutes (to allow any residual power to drain from any internal components), and then put it back in and restart.
But that's a just basic, maybe kind of thing. If you're still getting that modem crash error try clearing the system cache partition. You need to restart your J3 into its Recovery Mode to do this -- power off your phone, and then simultaneously hold down the Volume Up, the Home, and the Power buttons. When running in Recovery Mode it's a text only interface so you need to use the indicated keys to navigate through the different menus. Select the wipe system cache option (it will only take a few seconds), and then restart your phone like you normally to exit Recovery Mode. The system cache is completely different than app caches and it resides in its own dedicated partition completely separate from the user data partition so this does not affect any of your personal data.
If wiping the system cache doesn't help, you might want to look into re-flashing the stock ROM. Flashing a ROM is essentially the same as reinstalling the operating system on a computer. As far as Samsung devices, flashing a stock ROM won't wipe the data partition so your data should be safe, but this is something of a risky project so make sure your data is backed up. If you don't have a back up solution set up, do so now! Your J3 is having a problem so its even more important but you should have back ups no matter what the case. Do NOT rely on any smartphone to be a long-term, safe place to store files. Anyway, to flash a stock ROM you should be able to find one for your J3 Pro here:
https://www.sammobile.com/firmwares/
Do a search for your model of J3 Pro and your carrier, don't substitute as there is no mixing of ROMs, they're specific to each model. On the download page of your ROM, at the bottom there are instructions on how to flash it. (Note that sammobile intentionally restricts the bandwidth of its download servers so don't be surprised that it takes a long time.). Also, the Odin utility is a Windows application so if you don't have access to a Windows PC there is Heimdall, available for Linux, Mac, and Windows.
http://glassechidna.com.au/heimdall/
Another option if you don't want to use Odin is Samsung's Kies utility. It does make the procedure simpler but personally I haven't relied on it much. YMMV:
How to reinstall stock firmware without flashing by using KIES
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