This is a two part question and I apologize if they are stupid questions but just looking for clarification. Thanks in advance.

First question
Super Nova and Nova Corps Centurion have the same power to provide RCE to other friendly characters with nova corps keyword but its only +1. If you are using both of them would the other friendly characters with nova corps keyword get a +2?

Second question
Super Nova also has a power where you can give an unavoidable damage to each friendly character that has nova corps keyword and either increase your attack and damage +1 for each unavoidable damage or heal +1 next turn for each unavoidable damage. Does the rule of three apply? I thought maybe not because you are dealing unavoidable damge to raise stats or heal.

No. They each allow other characters to use RCE, but with a +1 Dmg instead of a +2. You cannot use two instances of RCE at the same time, therefore you can only get the +1.

The unavoidable damage is not restricted by the Rule of 3, by any means, but the potential stat bonuses it can grant are restricted by the Ro3. So, even if you give 4 clicks of unavoidable to friendly characters, you could only boost Attack and Damage up to +3.

Note that the Rule of 3 does not affect healing, however. So you could absolutely heal as many clicks as you dealt unavoidable in the previous turn.

First answer:
No. No matter how many Super-Novas and Nova Centurions you have (or combination of), it is still just +1 from the Ranged Combat Expert they grant.

Second answer:
Quoting the relevant part of the power: "1) Modify this character's attack and damage values by +1 this turn." Note the underlined part. So it modifies a combat value, and therefore is subject to the rule of 3.

Also note that RoT deals with the net modifier, so if something's modifying Damage value by -1 the extra +1 would negate the penalty.