For me if have gold ultimate the champion dragon because normally they are the hard annoying dragon to beat spending long hours to beat them. Because at the end of the day you only spend 30 gold compare to buying the final dragon which is 1,500 gold..
Also for short tournament like 3 days I advice don't spend too much time in a dragon to beat bec you might run out of time. Only good for 7 days tournament.
Cheers!!!