The 3rd, 5th, etc. are the notes within a scale. So, to use C Major as an example
1 2 3 4 5 6 7
C D E F G A B
The reason I said bare in mind where the root note is is because I assumed you had learnt scale patterns, therefore all you would have to do is to find the 3rd note by using the root as a reference. I've gotta go, but if you still don't understand, which wouldn't surprise me with my explanations, then I'll try and help again.
Don't worry too much about me, ignore me long enough and I'll go away.
