I have exactly the same problem.
I have bought the Samsung A3 2017 on august 2017 and since the last update at 11/6/18 my phone started to overheat and the percentage of the battery falls dramatically.. what should i do?
If you live near a Samsung Support Centre (I went to the one in London Surrey Quays) then try going there, that's what I did. Although when they plug the phone in and have a look at it, they'll probably say nothing's wrong - so insist that they take the phone away to their workshop. After doing that, my A3 was fixed a day after I gave it to them. Bear in mind that when the phone comes back from the support centre it's resetted so make sure to back up your data beforehand.
It's been half a year now since I went to have my phone fixed and it's working like new.
I'm assuming your device is still under warranty - if it's not then you might have to pay for the repair, but mine was free since my A3 is 1.5 yrs old, warranty lasts for 2 years. If there's no support centre near you I think you can arrange for your phone to be picked up from your house/work (if you live in the UK).
Hope this helps! I'd be interested to know if it does.