Have you considered bundling expansions like this together (either with the Dashboard -> Promotions -> Sales/bundles tab if you really need them to be available separately, or by offering a page with downloads for all three for $3)?
Since Itch doesn't have a cart, each separately-sold expansion will generate its own transaction, and you'll be charged PayPal/Stripe's fees on each of those transactions. The percent that you lose to PayPal/Stripe on each transaction will be dramatically lower if you sell more than one item at once. (It's also a little inconvenient to need to do a separate transaction for each item on the customer end)
3 expansions @ $1 each = $0.33 PayPal fee * 3 transactions = 33% of the money goes to PayPal
3 expansion bundle @ $3 = $0.38 PayPal fee * 1 transaction = 12.7% of the money goes to PayPal, with the percent getting lower as you bundle more items









