Most likely your lighting. The intense lighting in a small tank needed for the algae growth on live rock can heat the water alot more drasticly then a larger tank. You can try to vent the lighting fixture with drilling holes in the top. A table fan will also help. Keep room temperature as low as possible with an AC unit if necessary. The Goby sounds like a good fish for a "nano" reef. Clowns usually grow from 7.5cm - 15.5 cm. When they are babies it "may" be OK but it sounds like way to big of bio-load for such a small aquarium, despite live rock. Try the link below. I "think" that you may need a minimum of 10 gallons for all 3 fish. With that it is still a challenge.

Here is a link that might be useful: reef central