@vagabondpiper
Oppo Find 7 vs LG G3. Both run same 2.5 Ghz Quadcore Krait 400, snapdragon 801, and Quad HD. They indicated heat up on G3 only. But Quad HD needs muscles to run, i agree. Use of lower brightness may help.
http://m.gsmarena.com/oppo_find_7_vs_lg_g3-review-1103p6.php
Keep these thermal things stock. Its to protect your phone. Read the battery from both sides. It's mentioned there that battery temperature upto 40C is acceptable and do not expose battery to high temperatures greater than 60C. Now you know the reason, that batteries may also explode at higher temperatures.
http://www.spigen.com/lg-g3-case-slim-armor.
This is dual layered (soft+plastic) protective case, perfect for protection. But worst, when it comes to thermal conductivity (heat transfer ability). Don't use it, as you are already having overheating issue.
I used similar cases which overheated my S4 repeatedly and swelled the battery.
G3 has Corning gorilla glass 3 which is adequte for screen protection, but for extra protection, tempered glass protector be useful for front, rubber (soft) bumpers for sides and leave backside as it is for ventilation.