This appears to be poison ivy (Toxicodendron radicans).
Descriptions (Univerity of Michigan):
Distinguished by alternate compound leaves composed of three leaflets, the middle leaflet petiolule longer than the other two. The leaves of this woody plant are usually shiny, especially when new. Leaf margin varies greatly: smooth, toothed and lobed are all been observed. Leaf shape also varies: some leaves elliptic and others much more round in shape. Leaf color changes with season and age; young leaves in the spring often have a reddish tint, while mature leaves turn dark green, while leaves might change to orange yellow and red before falling in the fall. Toxicodendron radicans can grow as a vine, shrub, or sprawling plant; sometimes vine stems have a hairy appearance due to aerial roots that emerge from climbing stems
Long answer
As can be seen from the range maps in BONAP's North American Plant Atlas*, three species of Toxicodendron have been found in Ontario: T. radicans ([eastern] poison ivy), T. rydbergii (western poison ivy), and T. vernix (poison sumac). Well-known (by name) [Atlantic] poison oak (T. pubescens) does not occur above 40oN Latitude (i.e., North of the "Mason-Dixon Line") so can be ruled out. Of the three species present in eastern Canada, only the two species of poison ivy are trifoliate (i.e., having 3 leaflets). T. vernix is also grows as a tree so can be doubly ruled out.
*Note: USDA PLANTS Database shows greater range spread of all species (e.g., see here and here), but the species indicated to be found in Ontario does not change.
How to tell the difference between eastern and western poison ivy?
Different sources use different criteria. Let's explore:
Rhoads & Block (2000) "The Plants of Pennsylvania"
A. vines or erect shrubs; leaves trifoliate
- aerial roots usually present; leaflets not folded....................... T. radicans
- aerial roots not present; leaflets somewhat folded along the midrib..... T. rydbergii
Left: Aerial roots on climbing vine of T. radicans [Source] ; Right: folded leaflets of T. rydbergii [Source]
- To be fair, I've definitely seen T. radicans appear to have slightly folded leaves under certain circumstances, so I think this aspect of the key is suboptimal.
Weakley (2015) "Flora of the Southern and Mid-Atlantic States"
1 Leaflets 3, toothed, lobed, or entire; shrub or vine.
2 Fruits pubescent or papillose; leaflets entire, coarsely toothed, undulate, or round-lobed; lower surfaces of leaflets either velvety puberulent, sometimes becoming glabrate in age (T. pubescens) or glabrous (glabrescent or rarely pilose beneath) but with prominent tufts of tannish hairs present in the vein axils (T. radicans var. radicans).
3 Leaves sparsely pubescent (rarely pilose beneath), the apex and the lobes (if present) generally acute to acuminate; drupes papillose, scabrous or puberulent; plant a high-climbing vine or stoloniferous shrub; [of mesic, swampy, or dry habitats].......... T. radicans var. radicans
3 Leaves velvety puberulent (sometimes becoming glabrate in age), the apex and the lobes (if present) generally obtuse to broadly acute; drupes pubescent (becoming glabrate); plant a stoloniferous shrub; [of dry habitats, especially sandhills] ............. T. pubescens
2 Fruits glabrous (or very sparsely pubescent); leaflets coarsely toothed or notched (rarely entire); lower surfaces of leaflets glabrous to pubescent, but without tufts of tannish hairs in the vein axils.
4 Leaves densely pilose and velvety on the lower surface; leaves pubescent on the upper surface; pubescence of the leaves erect ................................................................................................................................. T. radicans var. pubens
4 Leaves glabrous to sparsely strigose on the lower surface; leaves glabrous on the upper surface; pubescence of the leaves appressed.
5 Leaflets suborbicular or broadly ovate, nearly as wide as long; petiole glabrous (rarely glabrescent); plant a shrub, the stems upright, entirely lacking aerial roots, not vining; fruits (3-) 4-7 mm in diameter .......................................... T. rydbergii
5 Leaflets ovate to lanceolate; petiole puberulent to densely pubescent; plant a shrub or vine, the stems upright or twining; fruits 2.5-5.5 mm in diameter ................................................................................................... T. radicans var. negundo
Clearly, Weakley goes into much more detail and includes lots of less well-known botanical terms as well as various varieties (subspecies) of T. radicans. A look at USDA PLANTS Database suggests of the varieties of T. radicans listed, only var. radicans and var. negundo are found in Ontario.
Of note, if we could see tan-colored tufts of hairs in the corners (axils) of the leaf veins on the underside of the leaves or fuzzy (pubescent) fruits, we could definitively rule in favor of T. radicans var. radicans. Or, if we could see aerial roots on the vines we could rule out T. rydbergii.
T. radicans can have either glabrous (not hairy) or sparingly hairy petioles, which doesn't help us decide between T. radicans and T. rybergii (not that we can easily see that detail in your images anyway.
I find leaf shape and border (margin) to be quite variable for all species listed (as Weakley hints at), so stopping at leaf shape alone always makes me feel like I'm giving an underwhelming answer. Despite this, T. rydbergii will almost always have coarse teeth/notches, which your image does not. (The ovate/lanceolate shape of T. radicans var. negundo would itself stand out as unique from the others, and again your specimens lack this leaf shape).
If I zoom into your image, I can see evidence of leaf hairiness (look around the border so that the contrast of the darker background enables you to see the hairs). This rules in favor of T. radicans var. radicans
Other common trifoliate plants or poison-ivy look-alikes:
