Statements: All snakes are trees. Some trees are roads. All roads are mountains. Conclusions:  Some mountains are snakes. Some roads are snakes. Some mountains are trees.

All snakes are trees. Some trees are roads.

Since the middle term is not distributed even once in the premises, so no definite conclusion follows.

Some trees are roads. All roads are mountains.

Since one premise is particular, the conclusion must be particular and should not contain the middle term. So, it follows that 'Some trees are mountains'. III is the converse of this conclusion and so it holds.

All snakes are trees. Some trees are mountains.

Since the middle term is not distributed even once in the premises, so no definite conclusion follows.