This is what I'm using in my Freedom Force based mini-campaign, and I thought I'd go ahead and post it for people's consumption. You could actually use it as an alternate to the 2e knockback rule if you didn't like that one too.
Note: This is set up so that when knockback is actually checked for, it will probably occur; that's almost certain if some of the optional extras are applied. That said, it'll rarely send someone so far back that they can't rejoin the fight.
The Basics:
Knockback is an optional effect of a critical hit. That does not mean that it uses up any of the other normal critical hit options (you still can add 5 damage or do any of the other normal critical results). I refer to it as optional since the attacker has the choice whether to do knockback or not (to avoid situations where this rule would otherwise mean a crit is actually detrimental to the person rolling it, a low mobility melee attacker for example). Knockback is only possible with attacks targeted at Toughness (which, in 3e, appears to be only damage).
To determine knockback, take the damage of the attack (including critical damage), and add 15 to determine the DC. The target makes a resistance check using Strength, Dodge or Flight ranks at his choice.
Ranks of Growth will directly add to Strength or Flight value for this purpose; half the rank of Impervious Toughness (i.e. the part that in 3e is actually Impervious) add to it too. Ranks of Defensive Roll will add to Dodge for purposes of this roll. Ranks of Shrinking come off any resistance.
A single degree of failure causes the target to be knocked down; each additional degree will cause the target to be knocked back one step on the distance chart, starting at distance 0 (that is to say, 30 feet). Targets do not take additional damage for hitting other objects during knockback, thought the objects will (equal to the rank of the distance); its possible under some circumstances such objects could include a person (usually they'll get a Dodge roll against the rank of the knockback to avoid this).
Other Related Rules
Instead of a normal attack, a person can attempt a Push: a Push takes a -2 on the attack roll, and uses the attacker's Strength to attempt knockback as indicated above with no need for a critical hit. An Advantage called Improved Push will eliminate the -2 hit penalty.
Unlike the case with normal knockback, a push can do damage to the target if he hits an obstacle.
Extras and other modifiers
Three relevant modifiers and powers effect knockback.
Improved Knockback: This flat extra adds 2 to chances of a crit only for purposes of checking for knockback per rank; this stacks with normal ranks of Improved Critical, but a 1 will still fail in any case, and attack that misses will not cause knockback no matter what the knockback check is.
Increased Knockback: This flat extra adds 1 point to the damage for purposes of the knockback check; it is not PL capped per se, but the maximum rank of it that can be purchased is 10.
Density: This is a Feature, and means the character is heavier and more dense. Per each rank of it, the character adds 1 to his Strength or Flight resistance checks against knockback; it also approximately doubles his weight, which may make it harder to pick up and throw him (of course it also can have some malign effects on his interaction with surfaces unless he flies all the time).
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