There are actually three main altitude groups for clouds, not just three specific altitudes. Here they are:

1. High Clouds: Found above 20,000 feet (6,096 meters). They are made of ice crystals and appear wispy or feathery.

* Examples: Cirrus, Cirrostratus, Cirrocumulus

2. Middle Clouds: Found between 6,500 and 20,000 feet (2,000 - 6,096 meters). They can be made of water droplets or ice crystals, depending on the temperature.

* Examples: Altostratus, Altocumulus, Altostratocumulus

3. Low Clouds: Found below 6,500 feet (2,000 meters). They are primarily made of water droplets.

* Examples: Stratus, Stratocumulus, Nimbostratus, Cumulus, Cumulonimbus

Keep in mind that these are general guidelines. The exact altitude of clouds can vary depending on factors like location, season, and weather conditions.