Order these clouds from highest (1) to lowest (3): stratus alto cirrus

The highest cloud would be cirrus. Cirrus refers to clouds which can appear at heights above 5 kilometers (16,500 ft) up until 14 kilometers (45,000 ft).
In the second place we have alto. These are the clouds which appear at altitudes above 2,000 meters (2 km), up until 6,100 meters (6 km), making them middle-altitude clouds.
In the end, we have the lowest clouds, which are stratus. They are low-altitude clouds, appearing at heights below 2,000 meters (6,000 ft). 
So: 
1. cirrus
2. alto
3. stratus