They have not actually seen the other two calves, but they have found tiny rhino footprints in the mud, which suggests at least another two animals have been born.
These calves are the first rhinos to be born in the last four years.
The World Wildlife Fund, which has been helping to study and protect the Javan rhinos, think this is a good sign.
They now hope to slowly build up the rhino population in the park.
Pictures by Yahya and Sumiadi, WWF & BTNUK