vRO 6 - input validation in user presentation

Hi,

I need to do some input validation in a user presentation, that will get imported later into vRA 6.

The user has to input some information in a form and there are 3 input fields. At least one of the fields has to be filled with input, so that the workflow can proceed.

How can I force at least one of the 3 fields to be mandatory? It can be any of the fields also optionally 2 or all. Can this be done without an action running in the background?

Thanks!