private double _a;
    new public double a
    {
        get { return _a; }
        set
        {
            SprawdzDane(value, b, c);
            _a = value;
        }
    }

public TrojkatMutable1(double a, double b, double c)
        : base(a,b,c)
    {

    }

TrojkatMutable1 tm = new TrojkatMutable1(2,3,4);

object.a = 10