Upgrade Camapaign - Multi Year Membership Group

We are doing an ‘upgrade campaign’ asking donors to commit to moving to a new membership level by stepping their pledge up over a three year period.  The donor will receive the higher level benefits while they are working their way up to the new level.  We’ll have to set up a separate membership group to do this and I was wondering if it would be better to have one membership that spans those three years or 3 separate memberships or not?  Anyone have experience managing the benefits and memberships for this kind of camapaign?

 

Cara Crocker