Because it is a third you will need three letter names C, D, E. This is same as using adjacent lines and spaces on the staff. So we know that it will go from C double flat to some kind of E. Next, because you want to build a Major 3rd you will need to count 4 half steps. Counting on the keyboard or frets on a guitar you end on D. However, this must be written as some kind of E because we want a third. E double flat is the same note on the instrument as D natural. 

The thing to remember is that intervals are made up of two things, 1) the number of letter names (lines and spaces on the staff) and 2) the number of half steps. The interval number always answers the first question. The qualifier M m dim Aug tells reveals the number of half steps. 

MusicGoals Eye and Ear provide drills to master this and gain fluency with intervals. The Music Theory section of the help file has examples with pictures and instructions on building intervals. Press F1 to open help, then look for intervals in the music theory section on the table of contents.