In Gravity Forms for your dropdown you will have a value. I found that as long as the value was one word it was easy to get it to be the default.
For you I would name each of your three things PlanA, PlanB, PlanC
Make sure show values is checked if you want it to be required.
Go over to the Advanced tab check the "Allow field to be populated dynamically" box and then for the parameter name enter a variable. I used default_value. Then on your different Plan names in your table you will need to add this. <a href="planForm/?default_value=PlanA">Plan A</a>
where "planForm" is obviously the link of your page and the default_value is equal to the value of the dropdown item you wish to be default.