choose only one 

no info
huge premine
didn't even change the Genesis block message

How to mine multiple coins on the samer rig? E.g. ETH with the 8GB cards and RVN with the 4GB cards? 

So Dogecoin is going to literally go to the moon. 

Directed my mates rig to Turtlecoin, nice project! According to https://www.cryptunit.com/device/570 each 570 should get 29.5 kH/s, but his 570 4GB only get ~20kH/s. Is it because his cards only have 4gb and not 8gb?

Miner still not linked in the OP, also no mention of special miner required when you released the coin on this forum. Having that info available on your discord server doesn't mean that you don't need to post it here. Launch was not fair at all and your coin will suffer the consequences. But with almost 10% premine this coin was doomed from the beginning anyways.

He probably sold already, so stop spamming your signatures. 

I don't believe in "coins" that require their users to shill and do social media tasks to earn an airdrop. Inface I would actively avoid such projects. If a coin or token is good and innovative, word of mouth will be enough to create some organic buzz, which is way more impactful anyways.

This morning I randomly had 1000 satoshis in my account and I have no idea why I got them. Ofcourse I bought some tokens with those but am still wondering where they came from. The roll history doesn't list them. I don't mind the free satoshis but am curious how and why this happened. 

If you mined a coin that nobody alse can see (no block explorer) or receive (no transactions "yet"), did you really mine a coin? 
Lots of empty promises on their webpage, their FAQ is eye-opening if you read between the lines.
No keys, no testnet, just some numbers they pulled out of their arse.

That's hilarious that nVidia themselves provided the driver that circumvents the mining throttle. Someone has been fired over this for sure. 
https://www.theverge.com/2021/3/16/22333544/nvidia-rtx-3060-ethereum-mining-rate-limit-unlock-driver

Are you sure it's not mining? The errors are only for fan speed and straps. My guess is that if you leave the miner running for a few minutes you'll see some results. Post your full commandline arguments. 

Power draw depends entirely on the graphics card and there are many that pull more than the specified 75W. But I don't know if risers that use e.g. USB cables are able to deliver 75W over the USB cable. I was under the impression that modern risers don't take power from the motherboard at all and use their power connector to deliver power to the GPU. A clarification on that question would be nice.
Molex-4-pin is safe if it's a well built adapter that uses original Molex connectors and thick enough cables. I wouldn't use flimsy 3$-adapters from China indeed.

I don't like these splitters as the 6-pin connector is designed for 150W and the 8-pin connectors are designed for 225W. So theoretically you could send 450W via a connector that is rated at only 150W. Bear that in mind.

If the PC has an onboard GPU, that is indeed the most sensible option, as those are usually good enough for desktop use. Obviously you can't game while you mine. Might be annoying to switch cables all the time though if you decide to do something GPU intensive for which the internal GPU does not suffice.

Type getmininginfo into the console. When I tried out this coin hoping for some quick blocks due to the low difficulty, hashrate stayed at 0, leading me to conclude that it was broken. Perhaps it's fixed now or it didn't work for a different reason.
Anyways, since OP didn't bother to clear up the "secret mining phrase" controversy and only chinese newbie accounts promote this coin I recommend to stay far away.

Well you could mount the cards with risers in a large PC case in a creative way if it has to be stealthy. There are very wide mining motherboards with 8 PCI-E slots where you can directly mount 8 GPUs, but they are weird, kinda expensive (compared to old used hardware) and can be problematic in Windows.
As for risers, just make sure to not buy the ones powered with SATA as they are prone to fire and other fails. Molex-4-Pin should be fine if your PSU can handle the load, otherwise go for risers with a PCI-E power connector and power them with a server PSU + breakout board. Performance and stability should be fine if they are built correctly. Buy from reputable mining shops instead of eBayor Amazon if you are unsure, slightly more expensive but they won't sell you shit.

Dogecoin has been added to PoloniEx Futures and can be traded at high leverages now. Not sure if this is the first exchange to do so, but I haven't heard of others. PoloniEx is #12 on this ranking https://coinmarketcap.com/rankings/exchanges/ with a 24h volume of 182 million $, so this is quite big news imo and the bigger players surely are watching.
https://poloniex.com/futures/trade/index/DOGEUSDTPERP
https://twitter.com/Poloniex/status/1372534909403688962

Cool rigs, the watercooled one looks really good.
There are also these chinese PSUs with 8 PCI-E connectors, can't comment on their reliability however: https://www.aliexpress.com/item/32869823311.html
And there are breakout boards that include an ATX connector but they are pretty expensive: https://www.parallelminer.com/product/zsx-breakout-board-16-ports-pico-24pin-atx-fan-hubs-100-gpu-mining-a-game-changer/
But the Seasonic should totally be fine for 3 1070s unless the board really requires all risers to be powered, which would seem stupid but who knows? But even then you could use Molex-4-Pin->PCI-E adapters. Would be a shame to let that quite expensive PSU go unused...

Googled around a bit, the slots are basically just risers and don't need to be powered if not in use apparently. Weird issue, maybe the slot you tried is screwed? If the GPU is known good and power is not the problem, it has to be the port, right? 

Did you try booting without a GPU? It should fall back to the internal GPU. Also there should be a jumper next to the BIOS battery, usually it's labelled CMOS reset. Switch its position, turn power on, then off and put it back in its original position. Voilá, BIOS reset. If there's no jumper, take out the battery and wait a few minutes and the BIOS will also reset to default.
I also second JayDDees recommendation about testing with a different GPU if available or testing the GPU in a different mainboard. But I wouldn't rule out power, these mainboards are weird af lol.
Cheers and good luck!