Question

How can I write about the spread of Buddhism citing 3 examples of evidence in 3 or 4 sentences?

Answer 1

The Silk Road was a route for the spread of Mahayana Buddhism
The Sea routes from India to South East Asia were a Route for Theravada Buddhism.

Explanation:

http://www.buddha101.com/h_spread.htm#Silk Road

This website talks about the spread of Buddhism and may be enough to use as an analysis.

You might want to find actual books on this to get that many references. Guns, Germs, and Steel by Jared Diamond talks about exchanges down the Silk Road and has the possibility a passing reference about Buddhism. The whole book is available without cost on line.

Wikipedia might give you a third. There should be others.

https://en.wikipedia.org/wiki/Qigexing_Buddhist_Temple_Ruins
https://en.wikipedia.org/wiki/Dunhuang
https://en.wikipedia.org/wiki/Jiaohe_Ruins
http://www.easytourchina.com/blog-v1142-8-giant-buddha-statues-in-china

Specific evidence includes ruins of monasteries, written evidence in China after the Warring States Period, Art work and Statuary in China and Conflicts with Confucian principals and banning of Buddhism.

The actual boiling down to 3 or 4 sentences I'd have trouble with its up to you.