Hi Jack.
Finding difficulty forming certain chords is a very common issue. It can be different chords for different people.
You do have a choice with D minor of either 3rd finger or 4th finger for the 3rd fret of the B string.
You do need to keep working at it but move on as you do so, making sure it remains in your practice routine. Why not try a different approach?
To improve the formation of a chosen chord (D minor for you here):
Hold your fingers near to but not touching the strings.
Touch the fingers where the chord is but do not press.
Once you have all three touching at the correct place then press them down.
Do not strum - this is a fretting hand exercise only.
Release the pressure after a few seconds but keep touching the strings.
Then move your hand away from the strings by a small amount. All fingers away.
Repeat the process.
Then, to improve changes to and from the chosen chord and other commonly grouped chords, repeat the above process with one alteration. After the final step of lifting all fingers away, the next cycle would be to move fingers over the chord that you are changing to. Once that chord has been done and fingers are lifted away, go back to the first chord of the pair.
Wash, rinse, repeat. Make this exercise last about five minutes.
Example - D minor chord plus some associated chords
1 minute - Dm alone
1 minute - Dm & Am
1 minute - Dm and C
1 minute - Dm and Em
1 minute - Dm alone
Practice daily and within a week you will be smashing it.
Hope that helps.
Cheers 
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