It's still not an ideal tool, but it's better than nothing. Periodically, images can be obtained where there are only two heads or more than three. Why this happens is a mystery to me.
Nevertheless, you can try, good luck.
(V.5.2) Illustrious version.
(V.5.1) A slightly modified version. Key tag: 3Hs. Additional tags: Three heads, 3 heads, three-headed, three_heads, 3heads, threeheaded.
(V.5.0) A larger version than the others. Use any of these tags: Three heads, 3 heads, three-headed, three_heads, 3heads, threeheaded.
(V.3.0) I tried to add more variety.
(v.2) is based on such creatures as cerberus, three-headed dragon, hydra.