MINION: SINESTRO CORPS:
When a friendly character with the Sinestro Corps keyword that's 75 points or more is within 8 squares, modify Sinestro Corps Recruit's damage value by +1.

QUestion:

1) does this mean that if I have 3 friendly character with the Sinestro Corps keyword that's 75 points or more, then my Sinestro Corps Recruit would have a damage value modified by +3 or would it be by +1?

It does not stack. You will only get +1.

I think it only modifies it by the +1.
Since its an ability that targets only himself, then I think he can only be affected by it only once.
I might be wrong tho.

I know that if it where, "target said 75pt character or more and modify his by +1" he is then bound by the rule of 3

This one is tricky for me

That's not the reason, actually. It is in the wording of the power.

What you have is a yes/no test with one result.
Is there a 75+ point Sinestro Corps guy nearby?
If YES then you get +1.
If NO then you get nothing and like it.

To have it apply multiple times, you would need language to spell out that it activates for each.
When a For each friendly character with the Sinestro Corps keyword that's 75 points or more is within 8 squares, modify Sinestro Corps Recruit's damage value by +1.

Remember that the rule of three doesn't cap anything until you use it. It also doesn't get rid of any modifiers when applied. They are still there, just not in play at the time you need them.

In other words, I can Perplex up my damage by +6 if I want. I just will never exceed a +3.