How to check if the user has filled out previous forms?

I have three events. The form for each event is different and complicated. I don't want to put three forms into one and mislead user.

All three events need to pay with different price. I want to give special discounts to users who register three events on the same day. Is there any mechanism to check whether user have registered event A and B. And give a special discount code when register event C.

Unfortunately, there are no options to check it a user has filled out any of your other forms. But, I can suggest setting up your forms to redirect users to next event based on their answers at previous forms. For example, you can ask a question if the user wants to register for other event and if their answer is YES, then redirect to a form with discount.

Here is a guide, which might be of help:

How-to-Automatically-Pass-Form-Data-to-Another-Form

Change-Thank-You-Page-URL-Based-on-a-Condition

You can also include a link to a discounted form in your Autoresponder email:

Setting-up-an-Autoresponder-Email

Hope this information helps. But, we will gladly assist if you need more help.