Emile de Maat
It says something like all dice are cancelled aftger a hit. Does this refer to evade dice, that it is impossible to apply evades?
No, the defense dice a rolled normally and can be used to cancel damage, as can the evade token.
The rule refers to the damage that is dealt. If the Ion Cannon hits (i.e. there is at least one uncancelled damage) then the target suffers one damage and receives an Ion Token. After that, all the dice are cancelled. This means that you do not apply any further damage.
If the result of the attack was three damage icons, then the Ion Cannon deals one damage and an Ion token - the dice are discarded, so no three damage.
If the result of the attack was three critical damage icons, then the Ion Cannon deals one damage and an Ion token - the dice are discarded, so no criticals.