I'm pretty sure somebody helped you out by now, but I hope I can add something that clarifies this subject alittle more.

To put it as simple as i possibly can, an interval is the distance from one not to another note within a scale. So, as you already know, from C to E is a third. Now, in order to figure out if it's major or minor, you need to think of C as being the root of the scale in this case and E being the third. So let's look at the scales C major and C minor below:

C major

C,D,E,F,G,A,B

C minor

C,D,Eb,F,G,Ab,Bb

C is the first note, and E is the third note in the major scale. By looking at these scales you can see that from C to E is a MAJOR third Why? Because that interval is borrowed from a major scale. So, what do you think C to Eb would be...?

Minor third. Because C to Eb is an interval of a third, and if you think of C as the root and Eb as the third, the only scale that makes sense with this is the C minor scale. Look again:

C,D,Eb,F,G,Ab,Bb
1,2,3

I hope this makes sense to you! But the most important part of any music theory is actually being able to hear it and apply it to your playing. Trust me. There's nothing worse than having all this knowledge, but when somebody asks you to show them through your playing you can't do it. Don't be like that lol. Plus, if you actually play these intervals and hear them, it will make tons more sense to you.

Take care!