Iet up the software for load balancing between 3+ machines?
I appreciate the info
The easiest solution here may be:
One Pi running Varnish at the "front"
That Varnish Pi proxying to two or more backend servers
SQL services running on another device
It's often worth looking at the backend somewhat before throwing more hardware at a problem. For example, I've seen some terrible SQL queries that developers wanted to throw more hardware at, when all that was needed was to tune the query.
Edit; You refer to Apache, but if the goal is squeezing peformance, the Nginx application discussed by the earlier poster may be a better option.