how to connect letters to composition

From what you've described, I'd do it with a script that would prompt you for the folder of 3D letters and would pull in the appropriate letters to make up the text contained in the selected text layer. Of course there are a lot of other details to take care of, depending on the final result you want, but it should be feasible.

Dan

I think the closest you could come would be to have a comp with all the letters stacked on top of each other. You'd need a copy of this comp for each letter position in your source text. The layer names would be the same as the letter (layers "A", "B", etc.). You would name the comps based on their position in the text string, so, for example, comp "Letter_0" would correspond to the first letter, "Letter_1" to the second, and so on. The letter layers inside these comps would each have an opacity expression that would retrieve the source text letter corresponding to letter position of that comp. If the letter matches the layer name, 100% opacity else 0%. Something like that.

That could work, but you wouldn't have any repositioning of the letters based on proportional spacing--your 3D letters would in effect create a mono-spaced font.

It would be a pretty ambitious application of expressions, but I think it could work.

Dan