Can one tenant be sued for entire unpaid rent if other tenants were liable as well?

A lease is merely a contractual arrangement.  If you lived there, rent should have been paid, unless you have a legal reason to withhold rent.

Yes.  You were a month-to-month tenant.  Just because there was no lease, there was still an obligation to make monthly rental payments to your landlord.  Additionally, even though there were other tenants who were also responsible for these payments you can be sued individually for the amount due.  This is known in the law as "joint and several  liability".  What you can do however, is to sue the other 2 tenants for the share that they were each responsible for paying and for which you may be found liable.  You can go to small claims court to pursue your claim against them.  In MA you can sue for up to $2,000 per claim (against each tenant).  Here's a link to a site that will explain how:  http://www.mass.gov/courts/courtsandjudges/courts/districtcourt/smallclaims.html