I remember how difficult it was to learn all the spellings for major and minor 3rds.
Trying to remember where to add sharps and flats was crazy! It was nothing like learning perfect 4ths and 5ths!
I think the biggest help in learning intervals in general was just learning how to count letters.
Like if you’re starting on C, and C is the “1st” note, then D is the “2nd” note, E is the “3rd” note, etc., you can really count the “number” part of intervals out on your hand.
How far apart is C to F? Use your fingers where C is 1.
C, D, E, F.
Four.
So C to F is a “type of 4th”. Is it a Perfect 4th? We don’t know just by using our fingers, but we do know that it’s SOME KIND of 4th at this point.
How about a 6th above C?
C, D, E, F, G, A.
C to A is a 6th.
The counting part, or the “quantity of the interval”, is pretty easy. It’s when you start trying to figure out the “quality of the interval” is where it gets tricky.
Minor 3rds are 3 half steps apart.
So going back to C we have: C to C#/Db. C#/Db to D. Then D to D#/Eb. But which one do we use? D# or Eb?
Remember that we need a THREE LETTER difference for a THIRD. C is 1, D is 2, E is 3.
So for a minor 3rd we need Eb, not that D# (because D# would only be a SECOND... an augmented second 😱) I know this is tricky, but with some practice, it will become clear over time!