<????????> element with exactly two children (child1, child2): $\stackrel{\text{child2}}{\text{child1}}$

<????????> element with exactly three children (child1, child2 and child3): $\underset{\text{child2}}{\overset{\text{child3}}{\text{child1}}}$

<????????> element with exactly two children (child1, child2): $\underset{\text{child2}}{\text{child1}}$