I’ve been using Justin Guitar lessons and I feel more comfortable with 3-2-1 for both D and A chords. Is that okay for the long term or should I stick with the traditional finger placement?
If a) it sounds good b) it’s comfortable and doesn’t cause any injury and c) it helps you move between chords easily, then you don’t need to change anything. If any of these things aren’t true, you should adjust. If they are, then you’re all good.
I started learning D and A chords using fingers 1 thru 3. When you get to Barre Chords, it’s helpful to use fingers 2 thru 4 for the A shape for Barre Chords. Some people also use the 3rd finger to press down all the strings for the A chord. It’s really up to how you want to play it.
Don’t overthink it. Just focus on playing the notes. Sometimes certain fingerings will make it easier to switch between chords. For example, you can play an A chord with just one finger, as long as the high e string rings out. Or when going from A to A7, you may only need two fingers. Focus on the notes you’re playing! 
For me, D=1,2,3 & A=2,1,3 is the best for the long run. You’ll use these shapes in many situations, and practicing them makes chord changes smoother. But that’s just my opinion.
Honestly, I’ve been playing for about 3 years and I’ve never really found Justin’s method useful. I usually Barre or use three fingers depending on the song.
Shan said:
Honestly, I’ve been playing for about 3 years and I’ve never really found Justin’s method useful. I usually Barre or use three fingers depending on the song.
I’m a complete beginner. How do you suggest I learn to play on my own? It would be really helpful to hear from someone with more experience.
If it sounds good, then it’s good.
Do whatever feels comfortable for you. You might even end up using both methods depending on the situation.
Eventually, you’ll want to learn different ways to play the same chords because you never know how the music might change. But for now, play however feels good and sounds good to you.