The next interval we need to look at is our Perfect 5th. Otherwise represented as 'P5'.
We can create this harmony by combining the first and 5th notes of any major scale. In this video we used C and G as an example. But for contrast, here are some examples in other keys.
F - F G A Bb C D E F. Our perfect 5th would be F and C.
G - G A B C D E F# G. Our perfect 5th would be G and D.
Bb - Bb C D Eb F G A Bb. Our perfect 5th would be Bb and F.
Just to be sure you understand the process of finding these intervals, I suggest you pick at least two other keys and identify the major 3rd and perfect 5th in those respective major scales.