words - How do 自他 triplets of related verbs work?

I thought this had been asked before (possibly by me), but I couldn't find it. I'm wondering why for certain verbs/words, instead of just a 自他 pair, there is a triplet (or possibly more) where two of the verbs are one type (自 or 他) and the third is the other type. Some examples:

• 他　→　縮【ちぢ】める　：　自　→　縮【ちぢ】まる　：　縮【ちぢ】む　→　both?
  
  
  
  • see @snailboat comment below - dictionary confirms. Exs.
    
    
    
    • 自　→　寿命が縮む　：　他　→　「ちぢめるの文語形」in dictionary def.
    
    
    
    
  
  
  
  
  


• 他　→　繋【つな】ぐ　・　繋【つな】げる　：　自　→　繋【つな】がる


• 他　→　緩【ゆる】める　：　自　→　緩【ゆる】まる　：　緩【ゆる】む　→　both? (same as above)


• 他　→　滅【ほろ】ぶ　・　滅【ほろ】びる　：　自　→　滅【ほろ】ぼす


• 他　→　含【ふく】む　・　含【ふく】める　：　自　→　含【ふく】まる

What's the deal with these triplets? Why are there two accepted verbs of one form for the same meaning? Are they somehow different? Is one of the two a classic/obsolete form like I talked about here? Does one of the two somehow rise to dominance over the other? I admit that I encounter 繋【つな】ぐ a lot less than 繋【つな】げる, but 縮【ちぢ】む seems just as frequent as 縮【ちぢ】まる to me.

Any other examples would be great to list too.