As thorc854 says, keep your other callbacks, especially with only two others. Firms expect to only get a fraction of the people they extend offers to. If nothing else, it gives you a chance to find out a little more about the other firms, and they may impress you enough that you keep them in mind for a lateral move in a few years.
Firms typically ask for an updated transcript after your 2L summer. They want to make sure you didn't just have a decent 1st year and are goofing off the rest of school, as it shows a potentially poor work ethic. As such, I don't think they typically care for an updated transcript after this semester's grades.
I expect any and all firms to do reference checks. At the very least they will call the human resources department to make sure that you aren't adding fictional companies and job titles to your resume. They will probably ask if there were any disciplinary problems and ask to speak to your manager if they have the time.
I don't expect them to do an extensive check and find out any jobs you left off your resume. They might do a few quick searches in criminal records, they could do a credit check, they could check if you are skipping out on alimony or child support payments, etc.