There are plenty of ways to get your rent reminder out to your three roommates. If you've got time, you could type out their email addresses every time. If you've got email savvy skills, you could create a group in your email client. But if you're smart, you'll use Fiesta.cc. It's a startup service that lets you easily create group email addresses that everyone can use—without ever leaving the "Compose Email" screen.
I've created groups in my Gmail account to easily email my entire family at once. But all that does is make everyone treat me like the personal listserv, sending messages along with a "Will you forward this to the group, please?" note at the top.
Instead, I'm going to start using Fiesta.cc, it's a free service that lets you create and email group addresses from inside your own email client.
It's really easy too: For your very first email, put everyone you want on your group list in the "To:" line, and make the subject your group name [at] fiesta.cc, as in "firstname.lastname@example.org" or "email@example.com." From then on, when you or someone in your group wants to email everyone, they just need to type "firstname.lastname@example.org" or "email@example.com" in their "To:" field.
And don't worry about ending up with a name like "family73129"; Fiesta.cc uses your email address to figure out which list to send to. That means that you and your friend (as long as they're not in your family group) can each have your own list called firstname.lastname@example.org, and fiesta.cc will never get them confused.
You can check out the website for more details, but you just need to open up an email to get started.