When I email Tor for bridges, I get three. I'm only going to use one, so why do I get three?
Basically convenience and/or choice.
Example: You've been using a particular bridge for a while - the first bridge of the three - but your ISP has now started to block it. You can try the second bridge without needing to send a further request email. In the best case, a small time saving. In the worst case, you're in a country that blocks Gmail/Yahoo, so even sending/receiving the mail is a pain. Or the Tor mail server is down for maintenance, and you have to wait an extra 20 minutes, etc.
Example: You've just received your email with the three bridges, but the first (and second) doesn't work because it's blocked by your ISP. You can try the second (or third) bridge without sending a further request email.
Example: If you have three bridges, you can choose the most suitable one to use. By performing an IP look-up, you can work out where the bridges are physically located (i.e. in which country). Perhaps connecting to a bridge in one country would look less suspicious to your ISP than one in another country. Again, you could continue to request individual new bridges via mail until you found one in a location you liked, but that just smacks of inefficiency.