Persimmon, the Roundup is not being sprayed, but painted on the cut surface of the ivy stem while the cut is still fresh (ideally only seconds after the ivy stem has been severed). So the Roundup doesn't get on the tree (you should be using so little Roundup that it doesn't even run down the stem of the ivy). And the Roundup which has been absorbed into the ivy breaks down long before the ivy rots, so no Roundup is released from the ivy roots into the soil. Four years ago I had two nandinas at the front of the house (planted by the Previous Owners) which I did not want. So I had them cut off at the soil and immediately painted the cut trunks with Roundup concentrate (41%). The nandinas did not resprout. The dwarf nandina was located between two dwarf Burford hollies -- the trunks were about 2' from the nandina. The hollies were not affected by the Roundup on the nandina stump; they have since grown together, covering the area where the nandina had been. The 6' tall, skinny nandina's trunk was about 3-4' from the trunk of a camellia (shorter than the nandina but much wider). The camellia suffered no ill effects from the Roundup on the nandina stump; it has since grown to cover the area where the nandina was ... I joke that the camellia is trying to eat the house! If you don't want to bother with a jar of Roundup concentrate and a paintbrush, a neater applicator is a small squirt bottle (ketchup, mustard, glue, whatever).

It's hard for me to tell from the photo. Maybe when it blooms you'll be able to tell for sure. I came across this link when I looked for poison ivy look alikes on Google. What about the photo near the bottom of fragrant sumac? Could that be it? Here is a link that might be useful: Poison Ivy and Look alike plants

Both are quite variable in leaf shape. Hortus III defines them with the same botanical name, Rhus radicans and or R. toxicodendron. Anyway, for sure #2 is poison ivy.

It looks like a box elder (Acer negundo) that may have been mowed off and is sending up shoots.