this is a very difficult situation, there is no cichlid IMO that can survive in under 30gals. these cichlids would be bolvians, gold/blue rams, apistos and shell dwellers. Pearls need atleast 45-50gals. So how long is temporary because i dont think this will work more than a day or to, very hard to filter it you would need to do water changes and add air pump. Cichlids normally like rocks but in 3 gal a rock would take up the whole tank. I really dont think this is going to work out well, any chance you could find a lage container or something that can hold water?
For very temporary.. a filter, 75 – 88 Deg F water temp.
For a proper tank some gravel, open swimming space and maybe plants, rocks and bogwood.
He will grow to 8 inches (avg adult size).. He will need like 100g tank. Hope you find him a home soon