Plans to be optional at Registration, but only one type should be selected

Is there a way of making the plans available at registration, where only one can be selected, but the choice should be optional?

That is: the registration can proceed (creating a 'free' user account) without purchasing a plan, but if the user decided to buy, they can only purchase one of the available three plans.

At the moment, I had to make the plans non-exclusive, otherwise the user is not forced to select a plan during registration. But this means a user can purchase all three if they wish.

I would like each user only to purchase one of the plans and only once for the year.

Yes, you've 2 ways you can go about this. You can make all your plans exclusive and offer a free lifetime plan like we do here at Joomlapolis or you can make all of your plans non-exclusive then under the Workflows tab add conditions so you can't combine them with your other plans. I recommend going the route of exclusive plans with a free lifetime option.