Parallel Cells
Not as big an issue as series batteries. Theoretically, you can view one cell as being 10, one meter long 'batteries' connected in parallel. Or 100, 10cm batteries, etc.
Discharge of paralleled batteries tends to be self balancing. As one battery provides more than its share of charge to the load, it's charge is reduced below that of is neighbors. And as its charge is reduced, its voltage drops and internal resistance rises, reducing its further contribution to the load until the others 'catch up'.
The problem arises when cell (or a section thereof) voltage rises with temperature faster than its discharge lowers it. That provides a positive feedback, causing that cell to discharge even faster, generating more heat. Now you have that thermal runaway.
Putting three paralleled cells in one can reduces the thermal gradient between them and reduces this effect. But the effect would still be present with only one cell per battery, given that the outer layer would lose heat to the container faster than those toward tthe center.