Unfortunately, three firewalls are worse than one. The extra firewalls actually increase the surface area of the system rather than decrease it, consequently the insecurities compound based on each firewalls assurance difference from 100%.

To simplify:

If firewall 1 = 99% assured (this is greatly simplied but bear with)

The systems firewalls are 99% assured.

If firewalls 2 and 3 are also 99% assured.

The systems firewalls are now 99% of 99% of 99% or ~97%... because each firewall compromise will compromise the entire system. (traffic can be manipulated at any point)

cheers,

catch