What is market Cap of a company if listed on 3 exchanges?

You need to look at the latest quarterly report to get a real picture of the shares outstanding or "the float", then multiply that by the share price.

Multiple exchanges in different currencies doesn't change that, if they are the same security.

If you notice in your example, $25mm Canadian dollars is ~$19mm US Dollars and Euro 16mm. But there is a reality that "the float" can include different shares, such as shares that are not vested yet.

We should be talking about the same shares of stock on three exchanges. It's not like they're trading one set of shares on one exchange and totally different shares on a different exchange. So no, don't add them together, just take any one of the three.

The three numbers should all come out to be the same amount of money if you converted between currencies. Assuming that you checked all three exchanges at exactly the same moment and got the currency conversion rates at the same amount, and the exchanges are all totally synchronized with each other and with the latest currency conversion rates.

I just checked and, going by exchange rates as of when I'm writing this post, they're pretty close. 16 million euros = 25 million canadian dollars and 16 million euros = 19 million US dollars. So the first two match what you quoted and the last is off by 1 million, which could simply be that exchange rates have changed a bit since you pulled these numbers.

Frankly I don't know how closely synchronized different exchanges are. I'll gladly yield to anyone with specific knowledge on this. If you make a trade on one exchange right now, do the other exchanges register it within seconds? Minutes? Can it take hours? I'd guess with modern communications its within seconds, but I'm just guessing.

Anyway, the short answer is, the 3 numbers should all come out the same, if you ignore the possibility of small delays in communication. Just pick any one.