Ramona and Verona sound kind of close with the three syllables, o, a, and r sounds.
However, I prefer Ramona. Verona just reminds me of the place and a maroon-ish color. It's still an okay name though. Ramona also has more nicknames.
I also like Thea. I actually have a classmate called Theo, so that's the first thing I think of. But besides that, Thea itself sounds like a strong name.
Of the three, I think I like Thea the best.
Thea is very pretty, but very safe and fairly conservative. Ramona and Verona each have a feel of their own, and to me, it's not a bad thing that Ramona is kind of a perennial eight year old girl. I like Verona, but I wonder if it'll get second-guessed as Veronica very frequently.
I like Thea least of the three, and I think the decision between Ramona and Verona would come down to details like surname and which side of the coin comes up. :)