I grew 3 of the 3' dahlias in a 16" pot. Plants grew fine. You may have an increased risk of fungal problems with the plants growing together like that. I did the same pot size with the 18" varieties as well.

I am debating if I will do the same thing this year or just put one in a smaller pot and set it on it's own.

Did your coworker tell you why s/he believed they weren't suitable for containers?

My coworker did say he hasn't grown Zinnias - but he thought that the root ball would be too big to fit in a reasonable size pot - and that if I put one into a small pot it wouldn't bloom. I'm so glad you didn't have problems with your zinnias!

If you were going to put just 1 zinnia plant (3') in a pot, what size would you pick? I appreciate all of the help - I am totally new to growing flowers.

Hmmmm... good question. That's one of the reasons I often put multiple plants into a larger container, more room for error ;-)

Don't take this as experienced advice as it isn't. This is just what I would try based on having grown the 3 footers in larger containers.

I would say 10" pot would be on the small end of what I would use. Is this plant going to be narrow and tall or more bushy? Top heaviness is something you may want to consider when deciding container size. You don't want something that easily blows over all the time.

I definitely don't want something that will blow over all the time. I think I'll go with 16 inch pots and try those for the zinnias and the dahlias. I have no idea how bushy the plants will get - I don't think I've ever seen zinnias or dahlias (this size anyway) in person. I've tried googling - searching this site, other garden sites, even wikipedia. I'm trying to learn, just not finding the information I'm looking for. Thanks again for your help. All advice (even "it worked once") is very welcome.