There's no way you can make a 3 millions items per hour with overworld portal spawn. Not in java. Don't know if possible in bugrock. I even have mechanist overworld portal farm and it produces over 90K but not 3 millions items. My farm works at y=0 with sliced portals. Then the ziglins goes to the over and get killed by tnt looting. I have reached one million items per hour by eliminating piglins spawns from the equation by vanilla methods. In theory you can make a 3 millions items per hour with my method but the amount of lag will crash any server. There's one farm that I suspect that is better than mine. It's hekate's but they didn't release any info of theirs rates and it uses rng manipulation in their version