Teaser 2812: Sum card trick

From The Sunday Times, 14th August 2016 [link]

Joe has nine cards, white on one side and grey on the other, with a single digit written on each side of each card. He gave them to Penny to arrange white-side-up to form three 3-figure numbers with the sum of the first two equalling the third. This was her attempt:

(2 1 9) (6 5 4) (8 7 3)

Then Joe turned the cards over one at a time to reveal

(9 8 7) (1 4 2) (3 6 5)

where the total of the first two did not equal the third. So he challenged Penny to arrange the cards so that both white-side-up and grey-side-up the third number was the sum of the first two, which she did.

What was her third grey number?