Well there are a lot of things that are needed when determining the real value of this trio. So more information is needed to really give you an answer you are looking for. A scan would be helpful.
Here is a link to T206.org that you can use to look up your cards based on condition, version, and which back each card has. It will give you a basic idea of what you can expect in terms of value.
Just for display - the Portraits for the 3 seems to be the most popular.
You may get a very slight bump for selling all 3 cards at time on E-bay, from someone that knows about the poem and is unwilling to chase the 3 cards on their own. But there is no real listed premium to the cards as a lot over seprate cards.
http://www.t206.org/value.php?card=79&back=17&grade=1