triple 777 and flinters I don't think so
I would say if you want to go with triple down the bore i would duplex the load using 10 grains if 4F first then the rest with triple 777 powder.
I my self only shoot 3F in my flinters and 4f in the pan 2 grains for a pan charge.
4F as a man charge wow ;I did that mistake last summer and drop 95 grains of 4F and alll I can say man what recoil.