3. (8 points) An Ultraman is fighting a group of small monsters. It is known that the Ultraman has one head and two legs. At the beginning, each small monster has two heads and five legs. During the battle, some of the small monsters split, with one small monster splitting into two. After splitting, each of the new small monsters has one head and six legs (they cannot split again). At a certain moment, there are 21 heads and 73 legs on the battlefield. How many small monsters are there at this moment?
【Answer】Solution: After removing Ultraman, there are 20 heads and 71 legs.
Assuming all are non-cloning monsters, then 20 heads correspond to 10 small monsters with 50 legs. Thus, there are $71-50=21$ legs less.

Each clone increases $2 \times 6-5=7$ (legs), $21 \div 7=3$ (monsters)
In total, there are $10+3=13$ (monsters)
Therefore, the answer is: 13