So when you've got an interval you usually mean two sounds that are separated in time. So like the iconic Jaws Melody dun-dan-dun-dan-dun-dan, those notes are separated by an interval that could be called one semitone, 100 cents, or a minor second, depending on who is talking.
Or in “Oh when the Saints Go Marching In,” the ‘Oh-when’ interval is two tones (four semitones), 400¢, or a major third, the ‘when-the’ interval is another minor second, and the ‘the-Saints’ interval is one tone or a major second. Adding those up we find out that “oh-Saints,” if you just omit the other words, is 700¢ or a “perfect fifth”, so “saints-Go” is a descending perfect fifth, -700¢.
Now you can play all four notes at the same time and you would still refer to these distances between the notes as intervals, but nobody is likely to describe this sound as a bunch of intervals. It is a “I(add 4) chord” in that context and the +100¢ interval between the major third and the perfect fourth is what gives it its spiciness.
So then you have to clarify whether you mean that we are playing one note first and then two notes together second, or are we playing all three notes at the same time, or are we playing all three notes separately.
If it's one and then two, or two then one, the higher note of the dyad will sound like the melody usually, and you'll reckon the interval between those two. People who have really well trained musical ears, instead hear the shift on the lowest note, but it requires training.
If you mean that all three are separated by time, then it's a melody. In this case these first four notes of “Oh When the Saints Go Marching In” would perhaps be described maybe as an arpeggiated major chord with a passing tone, same as I said earlier as “I(add 4).” I'm not actually 100% sure if that's the right use of the term passing tone or whether passing tones have to lie outside your scale or something.
If the three notes are played at the same time, that's a chord, specifically it's a triad chord. You might talk about the stacked intervals in that chord, a major chord stacks a minor third on a major third, a minor chord stacks a major third on a minor third, stacking major on major is augmented, stacking minor on minor is diminished, and there are suspended chords where you don't play either third, so sus2 stacks a fourth atop a second and sus4 stacks a second atop a fourth. So a lot of those have their own names, and some of those names get weird (like to stack a fourth on a fourth you might say “Csus4/G,” which treats the lowest note G as if it were the highest note but someone decided to drop it down an octave).