If you're willing to take the risk + accept the low expected monetary value, I'd say (3) will definitely let you learn most about starting a startup and finding

If you're willing to take the risk + accept the low expected monetary value, I'd say (3) will definitely let you learn most about starting a startup and finding cofounders. To that end, IMO ideally find a startup with <5-10 people so that you can get in early and learn from the ground up.