[Q]

Askan is right!
I made a test by modifying the intel projects:

Name := Counter - Intelligence Level 3
Description := Defensive preparations within an empire to prevent enemy intelligence operations. Counter-Intelligence will prevent attacks while it is in progress.
Group := Defense
Cost := 500000
Type := Intelligence Defense
Effect Amount := 3
Num Source Messages := 1
Source Message 1 := Our counter-intelligence operations have prevented intelligence attacks by the [%TargetEmpireName].
Num Target Messages := 1
Target Message Title 1 := Intelligence Project Defeated
Target Message 1 := One of our intelligence projects against the [%SourceEmpireName] was defeated by counter-intelligence defenses.
Source Picture := IntelSabotageByUs
Target Picture := IntelSabotageByUs
Number of Tech Req := 1
Tech Area Req 1 := Applied Intelligence
Tech Level Req 1 := 2

Changing the "Effect Amount" to 1 in all counter intel projects and my enemy needs now 480000 intel points per turn to defeat my attacking projects as I expected.
So a counter intel project 3 indeed multiplies your defense intel points by 3. And if you have a medium AI bonus and the standard defense modifier in der settings of 120% this gives the AI a factor of 3x3x1.2 = 10.8 for the intel defense. You need 10.8x more attacking intel points as a human player than the AI needs for his defense!! IMHO this is too much and I will probably keep the "Effect Amount" for all projects to 1.