When I buy parsnips they are all different sizes in the bag. How many cups do your 3 parsnips measure out to?

I don’t know how many cups but personally, I would just try to pick three roughly average-sized parsnips and if it seems like too much then just leave some out of the dish. You could even use four parsnips and that way you’ll know that you’ll definitely have enough. You can always leave some out and reserve them for some other meal. The parsnips are just a contrasting element - like acidic tomatoes in a green salad - so i think there’s a fair amount of wiggle room as far as exact proportions go.