I have a Galaxy S3 that I could use exclusively for dragon city (no phone, no other apps).

Does someone knows a way to make it reliable because right now, I can only gather food and gold, breed and hatch. Any kind of combat produces a crash.

Thank you !

Honestly I do not think the S3 has the power to reliably handle DC. But you could try uninstalling all unnecessary apps, disabling any pre-installed unnecessary apps. Changing the settings to use as little power as possible. That may work, but not knowing how much power it has, I do know my old Note 3 couldn't keep up as well.