>>3548394Honestly, if I HAD to pick one of the 3 and not shop used I would probably go with the D5600. If you're thinking of doing video get the SL3, but if you're looking for a jack of all trades, master of none type, it will be the D5600. The D3500 just isn't that good.
The SL3 does not support third party flashes. See pic related.
The D5600 is getting older and I wouldn't be surprised if they release a newer offering soon.
The D3500 is a snapshit camera for moms on vacation and teenage girls that are starting out.