24f:36r would be a little bit lower gear at the bottom end (0.66 ratio vs 0.6875 on your 3×9)

You won’t get smaller than an 11 on a ten speed cassette, so to have the ‘same’ ratio on a double crankset you’d need a 44T ring. Which won’t work, as a derailleur can only do 14T as a difference. But, how often (really) do you need your biggest gear ratio on 3×9?

So – if your needs are for the same at the low end and you can give up top end ratio, and assuming an 11:36 cassette as opposed to going for fancy 40T or 42T aftermarkt solutions, then I’d say a 24/38 double.

If you can compromise a bit at the low end, then a 26/38 or 26/40. But i don’t know if you often get 26/40, they’re usually 28/40.

I’d suggest 26/38; that’s slightly harder at the lowest end (0.72 ratio) and at the top end you’ll have 38:11 i.o 44:11. But 38:11 is almost the same as 44:13 so in effect you’re only losing your very top gear.

And yes, if you’re on a 29er, then the gearing comes out about 10% harder than the same combinations on a 26er once you account for the difference wheel size makes on the total gear inches.

What jonv said

To elaborate slightly, I’ve been running a 24/36 chainring setup with an 11-36 cassette for about 12 months or so now, after conceding that a single 32T ring was just a bit much for me on some of the steepest climbs. On a 26″ bike I found I never used the bottom 24/36 gear at all, and rarely used the next easiest 24/32 gear, but occasionally felt like I could do with a slightly taller gear than the 36/11 tallest it had. I’ve been using the same setup now on a 29er for a couple of months, and I find it pretty much perfect for every eventuality. With the 10% taller gearing caused by the larger wheels, the 24/32 2nd gear gets used a lot more, and I occasionally drop into the 24/36 on the very longest/steepest of climbs. At the top end, the 36/11 top gear is pretty much as tall as I need.