Oh, you're right.
In 2.50/2.73 there is no validation.
I tested in 3.x.
What you can do:
You have questions of type radio.
As a last question take a question of type "short text". Here Q1a, Q1b and Q1c
Relevance equation:
(!is_empty(Q1a) and !is_empty(Q1b) and !is_empty(Q1c)) and (Q1a!=1 OR Q1b!=2 OR Q1c!
)
You see the correct answers are 1, 2 3
Therefore this question is only shown if all radio questions are answered AND there are wrong answers.
The validation of the question is the correct case:
Q1a==1 and Q1b==2 and Q1c=
Now you remove the textt field of this question by setting a css class "mytext" and adding this to your template.css
.mytext .answer-container {display:none;}