Left photo - looks like Acer negundo, box elder.
Middle photo - looks like Clematis terniflora, Sweet Autumn Clematis.  This is an invasive plant and do not recommend planting it. It grows vigorously over other vegetation that blocks sunlight to other plants underneath. Spreads by wind dispersed seed.  
Right photo - Looks like it may be a hickory or ash seedling. Cannot say for sure.

All plants may have the potential to cause Irritation, allergic reactions, or sensitivity. We are all different and exposure to certain plants may affect some and not others. 
 mh

Thank you so much! The leaves are opposite on the seedling so I believe it is some species of ash because hickory's do not have opposite leaves correct?

Do you know of any resources for noxious/poisonous plants in Maryland? I'd like to do some research as a colleague had a blistering reaction to a plant and he said it wasn't poison ivy or sumac. As you stated though there is always the possibility he is just allergic to something.

Hickory - alternate compound

Here is an online publication on 100 Poisonous Plants of MD.  https://extension.umd.edu/sites/default/files/_docs/programs/woodland-steward/EB314_PoisonousPlantsM...
mh

Also, something else to note.  There are some stinging caterpillars out this time of year such as the saddleback caterpillar among others that can cause some skin irritations. A possibility, if not wearing gloves in the garden.
mh