Well, I think it really depends on a few things.

1. Are you wanting to keep the different irises separated, ie are they maybe 'named' cultivars and you don't really want them to all run together, making digging and dividing later more difficult to keep some of all of them?
2. What kind of a look are you going for? Individual clumps of the same cultivar, or else a mix of several different ones?
3. How soon are you willing to dig and divide them the next time? If planted closely, they will need divided again sooner than later.

Taken from the FAQ linked below...
How do I dig, divide, and transplant bearded irises?
.....snip.....
If you have the room, plant your rzs about 2' apart with no other plants nearby to overgrow them and compete for soil nutrients. You can also plant 3 separate rhizomes in a triangle 1' apart with the leaf fans facing outwards to produce an instant clump effect.

Here is a link that might be useful: How do I dig, divide, and transplant bearded irises?

thanks for you quick response - very helpful

mr toad

You're welcome.

Sue

I plant three of the same ones closer together- eight inches, but I have to divide often as a result. I like them in separate clumps. I think they look better.
Renee