How difficult is it to identify Asters as to cultivar?

Your tall lavender one could be Aster novae-angliae, now called Symphyotrichum novae-angliae

I have one like it, from Annie's Annuals called "Skyscraper'. http://www.anniesannuals.com/p...

I believe you are correct Sue. It does get really tall. The other two were given to me by Diana, she lives here in Dallas, OR.
I sure love them.
Going to add them to the Aster page since I don't know for sure what the cultivar names are.

The pinks ones have more the look of Aster novi belgii , the bright pink one looks like 'Winston Churchill' although I'm seeing big variations in the colour of that, there are several which look the same as yours.

http://www.phengels.fr/jardin-...

http://www.gardenworldimages.c...

http://www.tadine.ca/fleurs/as...

Aster novi-belgii 'Little Pink Lady' looks close for the lighter one..

http://www.fabfolia.co.uk/plan...

A page of pics on a grower's site to browse. It looks like yours are not the current varieties but it's nice to look at them to see the different varieties.

http://www.hogendoornholland.c...

http://www.fotosearch.com/phot...

Little Pink Lady and Winston Churchill are both older varieties and probably widely sold. There was once fewer varieties than there is now!

http://www.fabfolia.co.uk/plan...

I'm going to email Diana and see if she remembers the names of the two pink ones.