[WEB SECURITY] SQL injection and N tier Architecture

[Infosec]

Hello,

I need to know how the different architecture will effect on application security.
For example:

A. Web server and Database server
B. web server , Application server and Database server

How much the above architectures secure from SQL injection?

I know multi-tier is more secure, but I need more explination.
Multi-tier is more secure, but still doesn't prevent SQL injection, isn't?


Regards,