9% of 3 is easier to estimate because you know it’s “almost 10% of 3”. Or, since 10-1==9, you could think of it as (10% of 3)-(1% of 3) and get the right answer using some other shortcuts. Humans being generally pretty good at base10, this is easy to figure out in your head as (0.3 - 0.03) and get 0.27.

Or, you could do what another commenter suggested and “3% of 9” can broken down as (3/100)•(9/1), becomes, (3•9) / (100•1), becomes 27/100, becomes 0.27. And that can be simplified as xy/100.

Different tools for different jobs. Base10 tricks are good for stuff like figuring out, say, a 15% or 20% tip, because you can easily figure out a 10% tip just by moving the decimal one space to the left, and add half of that (for 15) or double it (for 20). Or half and half again for (almost) 18%. xy/100 is a good trick for figuring out small percentages like sales tax (unless you’re in a place like Mass where it’s 6.25 and you gotta change it now to 625y/10000. At that point I’d just estimate at 6 in my head, or if I had to solve it mentally do (6y100) + ((1y100)/4).