That's a very good question and any aws engineer should know about it.
Basically the three tiers in a 3-tier architecture are as below :
Web ( Public Facing)
Database ( Should be private as it may contain confidential data )
App ( can be public/private)
Now next thing is making sure that all tiers are made highly available, highly secure, architecture should include logging and monitoring, auditing etc.
For more details , please read the below article