YES
* Step 1: FromIts YES
    + Considered Problem:
        Rules:
          0. f2(A,B,C)   -> f2(-1 + A,B,C)   [A >= 31] (?,1)
          1. f2(A,B,C)   -> f300(A,-1 + B,C) [30 >= A] (?,1)
          2. f300(A,B,C) -> f2(A,B,C)        [B >= 21] (?,1)
          3. f300(A,B,C) -> f1(A,B,D)        [20 >= B] (?,1)
          4. f3(A,B,C)   -> f300(A,B,C)      True      (1,1)
        Signature:
          {(f1,3);(f2,3);(f3,3);(f300,3)}
        Flow Graph:
          [0->{0,1},1->{2,3},2->{0,1},3->{},4->{2,3}]
        
    + Applied Processor:
        FromIts
    + Details:
        ()
* Step 2: Decompose YES
    + Considered Problem:
        Rules:
          f2(A,B,C)   -> f2(-1 + A,B,C)    [A >= 31]
          f2(A,B,C)   -> f300(A,-1 + B,C)  [30 >= A]
          f300(A,B,C) -> f2(A,B,C)         [B >= 21]
          f300(A,B,C) -> f1(A,B,D)         [20 >= B]
          f3(A,B,C)   -> f300(A,B,C)       True
        Signature:
          {(f1,3);(f2,3);(f3,3);(f300,3)}
        Rule Graph:
          [0->{0,1},1->{2,3},2->{0,1},3->{},4->{2,3}]
    + Applied Processor:
        Decompose NoGreedy
    + Details:
        We construct a looptree:
          P: [0,1,2,3,4]
          |
          `- p:[0,2,1] c: [1,2]
             |
             `- p:[0] c: [0]
* Step 3: CloseWith YES
    + Considered Problem:
        (Rules:
          f2(A,B,C)   -> f2(-1 + A,B,C)    [A >= 31]
          f2(A,B,C)   -> f300(A,-1 + B,C)  [30 >= A]
          f300(A,B,C) -> f2(A,B,C)         [B >= 21]
          f300(A,B,C) -> f1(A,B,D)         [20 >= B]
          f3(A,B,C)   -> f300(A,B,C)       True
        Signature:
          {(f1,3);(f2,3);(f3,3);(f300,3)}
        Rule Graph:
          [0->{0,1},1->{2,3},2->{0,1},3->{},4->{2,3}]
        ,We construct a looptree:
          P: [0,1,2,3,4]
          |
          `- p:[0,2,1] c: [1,2]
             |
             `- p:[0] c: [0])
    + Applied Processor:
        CloseWith True
    + Details:
        ()

YES