How to convert 9 bit binary to BCD for 3 Seven Segment Displays
The modern technique would be to use a microcontroller to perform the conversion. Of course, you'd need one with (nominally) 20 bits of IO - 9 in and 11 out. (hint - what is the maximum value of 9 bits binary). You can get away with fewer IO lines if you add external circuitry: multiplexers on the input and latches on the output.

The old-school way is to use dedicated ICs, specifically the 74185. Of course, you'll need 6 of them, they're no longer made (although you can find them on eBay) and they need a lot of power, but it's doable.

The intermediate technique is to use a couple of PROMs. A couple of 2716s will do you, but you'll have to do your own programming. They're cheap enough although they take up some board space. This is the approach I'd use, but you can make your own choice.