Does it make sense to buy the 3" pan if I already have the 2"? I read one of the site articles about collaring a pan but too afraid to try it.

I really think it depends on how much you would be using it.

If you just need a taller cake once or twice, I'd either make another 9x13 and put on top or tort the one.

I've never collared, but good luck to you whatever you choose.