Her trait says. If Captain America is being carried modify the carrying character's speed value by +3.
My question is if someone like Dr. Strange or Renet Tilley is carrying 2 do they get a +6? If so how would the math work? I know there is a maximum 3 bonus but what if Renet is carrying 3 characters? Would the bonus be +6-3 so net +3?

I believe the rule of 3 would cap that at a total of +3 no matter how many you carry.

First off, Cap only grants a single +3 to the person who is carrying. Second, you're correct that you get a maximum of +3 due to the rule of three's.

Let's say you're running two Samantha Caps and Renet carries both. The sum of all modifiers cannot exceed 3. It would work like this:

Renet's speed value = 12
Penalty for Carrying -2 = 10 (this changes with the new rules)
Increase from first Captain America +3 = 13
Increase from second Captain America +3 = 15 (maxed due to rule of three).

You would end up with a 15 movement. Does that make sense?

Yes. I was thinking this was how it worked. Just wanted to make sure. Thanks! Some tried to say rule of 3 meant modifiers but I was sure it meant you couldn't go 3 over printed dial. So you could perplex someone 5 times to negate your opponent perplexing down an ability.

It does mean modifiers, not printed value. You could have a printed 16 defense, have your defense modified up 3 times, then when you are attacked, you use defend from your adjacent friendly character who has printed 19, and you now have a 22 defense against that attack. All that is limited to 3 is the net modifier. You could have a power that says 'modify damage by +4', you would only net +3, but a -1 from your opponent would net no effect as you would still end up with +3, it just means your opponent has to come up with -2 before they make a difference.

Thanks this helps alot.