Warning: I have no experience with Informix, answer is based on general observations
When specifying parameters the database will need to know the type of each parameter. If a parameter occurs in the select-list, then there is no way for the database to infer the type of the parameter. Some database might be capable of delaying that decision until it actually receives the parameters, but most database will need to know this at parse time. This is probably the reason why you receive the error.
Some databases - I don't know if this applies to Informix - allow you to cast parameters. So for example:
SELECT CAST(? AS VARCHAR(20)), CAST(? AS VARCHAR(10)), CAST(? AS VARCHAR(5)) FROM ...
In that case the database will be able to infer the parameter types and be able to parse the query correctly.
With this I do assume you are not trying to specify columnnames for the select-list using parameters, as that is not possible.