It's an Ecoflow Delta 1800, right? The generator part threw me off - it's not a gas-fired generator, it's a 'solar generator', right?
I was curious because I think they're great so I bought a Delta Mini. It's fantastic.
So your panels are 30' one way to your delta unit? I got the pigtail too - I use it with a 25' 10g hookup wire to my 160W solar panels - works fine, they peak out around 150w charging on the delta unit.
The 3 - 100W panels in series may well over-volt your charging parameters for the mppt controller in your delta unit. So you'll connect them parallel, or get a fourth one and connect them 2s 2P. IIRC the '10A' max is based on the ~52V of the delta batteries, so it's best to use watts, and I'm pretty sure your delta can accept 400W, right?
I would use 10awg wire, either PV cable if it's exposed, or simple pvc insulated hookup wire as I did, and plug into your supplied pigtail.
You could also easily just make up a PV connect wire to a xt60 connector as well. You'll get a little voltage drop with your low voltage, but do-able. And why I would consider setting up a way to combine panels for more voltage - less drop over your cabling.
I also use my portable panels to plug into an SAE port in the side of the RV that connects to an mppt solar controller for the house batteries. So I got a SAE adapter that connects my solar extension cable to the Delta solar charging harness. All good.
Oh, and if your have smaller wires they become a bottleneck for your current flow, but not a deal breaker if they're short. They'll carry the current, but the higher it goes the more they'll create resistance and voltage drop. The ecoflow cables and connectors are pretty darn good IMO.