Filling cells A1 with 1, A2 with 11, and A3 with 21 will certainly let me autofill all three rows accordingly in one swipe and match what's seen up top... but that requires me counting and working out what the cells A2 and A3 are.

I was hoping to skip that step, and be able to type in 1 and have Excel work out itself what goes in each cell after that, including what the cells at the start of row 2 and 3.

Just so I can bypass using my brain and saying "Okay, there's 10 cells in the top row. Therefore the next row starts with 11 (and type that in). Therefore the next row starts with..."