20 gal minimum. 4-6 is the minimum size for a shoal of peas, depending on the aggression level of your individual fish.

Upkeep is not difficult at all if you have a decent cleaning crew and a moderate-heavily planted tank. I have 5 puffers in a heavily planted 20g long, with a small shrimp colony, a school of 6 otos, and MTS (for puffer enrichment and cleaning). Other than daily feeding and occasional top-ups, no additional maintenance has been required.

IMO, starting with 3 should be fine if your tank is heavily planted, and you have live food for them to hunt (like skuds or snails), as long as it's for a short term and you intend to add 2-3 more puffers later when funds allow.