Having done a couple 20s10p pack builds (using the same Molicel 21700) with the copper/nickel sandwich method, this is what I came away with regarding the process and a couple months of daily hard (ab)use -
I used .1mm copper, .15mm and .1mm nickel. I built the spot welder I used, and it was quite powerful. Enough to vaporize a single .15mm nickel layer on a cell, and once it actually blew straight into the cathode of a junk test cell, just nickel again. With the copper in place (one sheet, shaped and covered the entire parallel group), the overpowered spot welder became a suitably powered spot welder once I adjusted the timer relay to deliver two rapid pulses, 10ms each, instead of a single 20ms pulse. The welds are solid, but that's the limit of what that welder could/should do. The best welds came with nickel plated steel strips. You WANT enough resistance that instead of conducting enough to spark, your probes transfer enough heat generated by highly resisted amps to penetrate the copper and create fusion with the stainless steel cell can.
I sincerely do not see how a triple layered "sandwich" would be feasible for several reasons. That's a lot of layered metal. Initially I imagine nightmares happening trying to get the stack of copper/nickel thrice high lined up to weld to the cell. Seems highly susceptible of a slip accident, then the stack spreads over wherever it spills... like the cell group next to it. Even if that was successful and you built it somehow, thats also a lot of metal layers hopefully have a penetrated fusion through all layers, if the top layer pops off and slides down the pack a bit, nightmares.
The spot welder to truly do that job correctly would be brutally powerful. Not a kweld, I'd guess. Not any device or method regular folk could afford perhaps. Sunstone Welders, maybe. Check out their stuff.
.3mm nickel is hefty af, I know with certainty my spot welder would not be able to truly fuse .1mm copper underneath .3mm nickel. At its highest performing, no way.
I'd almost be willing to suggest you select an entirely different battery construction method and cell choice if youre hell-bent on pulling several hundreds of amps constantly. EV WEST has some new LG CHEM 63ah pouch cells that can deliver some substantial power. Getting EV grade pouch/prismatic cells brand new is not something that happens in the wild, but you may have already purchased your Molicel's.
I suggest you get the most hotdoggin' spot welder financially feasible. Get several small samples of copper sheet increasing in thickness. .15mm, .2mm, 2.5mm, etc... Then get some .1mm - .2mm nickel. Test weld all thickness combinations until you have the heaviest/thickest of copper that still enable a sound weld. Remember, the nickel is primarily used to facilitate the weld and the copper carries the amps, so if you can slightly increase the copper thickness, you greatly increase the headroom of current you can draw while the pack is in use. Increasing the nickel thickness is arguably negligible in that regard, and will only make the welding process more difficult.