struct MyStruct {
    uint a;
    uint b;
    uint c;
};

struct MyStructs {
    uint count;
    MyStruct data[];
};

StructuredBuffer<MyStructs> sb;
StructuredBuffer<MyStruct> o;

groupshared MyStruct s[128];
static const MyStruct deflt = { 1u, 2u, 3u };

[numthreads(128, 1, 1)]
void main(uint id : SV_GroupIndex)
{
        s[0] = deflt;
        uint count = sb.Load(0).count;
        MyStruct ms = id > count ? s[id - count] : sb.Load(0).data[id];

        InterlockedAdd(o[0].a, ms.a);
        InterlockedAdd(o[0].b, ms.b);
        InterlockedAdd(o[0].c, ms.c);
}