mc_gross = Amount for single payment in currency of payment basket. Full amount of the customer's payment, before transaction fee is subtracted.

mc_amount1 = First amount of payment subscription in currency of payment basket

mc_amount3 = Recurring amount of payment subscription in currency of payment basket

If you need to send the full computed amount they paid you'd use mc_gross. If your software stores tax separately then subtract $paymentBasket->tax from $paymentBasket->mc_gross. Suggest reviewing our various analytic plugins for CBSubs for further examples.

Do you mean that mc_gross will include also any discounts (positive or negative) ?

Yes, it's the gross amount. mc_gross is typically what you should be sending to your software as it's the actual amount they will pay. When dealing with recurring subscriptions then their first payment is in mc_amount1 and their next recurring payment will be mc_amount3.

I remember having to compute with discount_amount a few years ago

If you want mc_gross without discount_amount then you'll need to do that math same as if you wanted mc_gross without tax. Note there's first_discount_amount and discount_amount depending on if it's first payment or not and depending on promotion configurations.

You can also itemize by calling loadPaymentItems function on the basket to return a list of basket items, which have a getPrice function for returning their individual item prices.

Yes, typically you'll only need to bother with sending mc_gross and minus tax if you don't want to include tax or want to send tax separately.