Attacks of Opportunity and Combat Reflexes
You cannot make multiple attacks of opportunity against a single target in a single round unless you have Combat Reflexes Expanded Mastery 7.
Click on the link above for more detailed discussion of Attacks of Opportunity in Iron Heroes.
Combat Reflexes 3
If you have this feat, then if an opponent exits a square you threaten, you may make an Attack of Opportunity. This applies even if they are tumbling, withdrawing, taking a 5-ft step, or using a feat or class ability that normally prevents Attacks of Opportunity. The only exception to this is Mobility 10. That feat's benefits override those of Combat Reflexes 3.
Remember that unless you have Combat Reflexes 7, you still cannot make multiple Attacks of Opportunity against a single target in a single round.
If you have Combat Reflexes 3 and you successfully Bull Rush someone, then when they leave a square you threaten, you can make an Attack of Opportunity against them. The normal limits for number of attacks of opportunity still apply.
If you feel that Combat Reflexes is too powerful, I suggest the following house rule:
If an opponent enters a square you threaten and you would not normally gain an Attack of Opportunity against them for this movement (for example, because they are using the Tumble skill, or Mobility Expanded Mastery 3), you may make a Base Attack Check opposed by their Base Attack Check. If you win, you make an Attack of Opportunity.
Combat Reflexes 5
If you have Combat Reflexes 5 and an opponent enters a square you threaten, they are considered to have moved 10 ft through your threatened area, as that square is difficult terrain for them (unless they are using a skill or other ability that allows them to treat it as normal terrain). So if their speed was 30 ft, they would provoke an Attack of Opportunity from you for entering any square you threaten.
Combat Reflexes 8
If you have this feat and you make multiple successful Attacks of Opportunity against a single target during a single round (e.g. through having Combat Reflexes 7) then you reduce his speed by 10 ft. per successful Attack of Opportunity. The penalty still only lasts until the beginning of the target's next turn.