What do you do when you have more than one company showing interest in you?

Last week I emailed my resume and portfolio to 3 different design firms and left a message for a 4th, since I’m seeking a summer internship. On Monday I was contacted by one of the companies wanting to know how many hrs. I needed, when I could start, etc.; I have an interview with them on Tuesday.
But now I just got an email from the 4th company I reached out to, wanting to know if I could send them a resume. Do I still send it to them although I’ve already got an interview?