Explanation:

The Shaka Era: The origin of the Shaka era, which is 78 AD, is traditionally attributed to the Kushan king Kanishka. While the actual historical aspects might have nuances, traditionally, it is indeed associated with the Kushanas.

Kanishka's Empire: Kanishka was the most famous of the Kushan emperors, and under him, the Kushan empire expanded to its largest territorial boundaries. His empire indeed included vast territories, ranging from Central Asia to northern parts of India. Important cities like Varanasi, Kaushambi, and Sravasti were under his rule.

Charaka and Asvaghosha: Both Charaka, an important figure in ancient Indian medicine, and Asvaghosha, a Buddhist philosopher, poet, and dramatist, are traditionally believed to be contemporaries of king Kanishka. Asvaghosha was a significant Buddhist scholar during Kanishka's reign and played a pivotal role in the spread of Buddhism during this period.

Given the above explanations, all three statements are correct.