I have several different Panicum cultivars in my garden, including a couple of the ones you list. I seldom cut them down in the fall. Occasionally, a heavy wet snow or an ice storm will lay them low, in which case, I cut them down right away, but most years they all survive the winter intact and upright. It all depends upon your weather. It won't bother the grass to be cut down any time after dormancy sets in.

Spacing is a personal choice. If you planted these three within 2 feet of each other, they will be joined at the top in 2-3 years. I wouldn't plant 3 different cultivars together, but if I did, they would be at least 3-4 feet apart. A single cultivar of ornamental grass is a specimen, and needs to stand alone from other grasses.

Thanks for responding to my question Donn.
I think the Panicum grasses are the prettiest.
I have recently really started liking grasses, so graceful.
Mine are definately far enough apart that they will not join together at the top in 3 yrs.
I think I am going to buy 2 more of the Shannondoah (sp) for my front yard garden for a graceful backdrop for my flowers.
Thanks again :)