So, some pokemon have 3 stage evolution and some have 2. Are 3 stage evolution pokemon stronger than 2 stage pokemon? In pokemon game series, of course.
It depends on the Pokemon, here you can see all the base stats of every Pokemon in any evolution stage.
You can see other generations by clicking on the links in the most above section of the post.
No, a 3-stage evolution does not necessarily make that Pokemon stronger than a 2-stage evolution. Take, for example, Butterfree, which is a 3-stage, and Lucario, which is a 2-stage. Butterfree has base stats of 60/45/50/90/80/70, a grand total of 395. Lucario has base stats of 70/110/70/115/70/90 for a grand total of 525. And that's not even the Mega Evolution, which is even stronger than that. This shows that a place in an evolution chain does not denote higher or lower strength in any way.
On average, the final evolution of a 3 stage Pokemon is stronger than the final evolution of a 2 stage Pokemon.
Average Base Stat Total of Fully Evolved Pokemon, by Stage
I pulled data from bulbapedia here, which lists the stats of all fully evolved Pokemon, and threw it into a spreadsheet. This gives me the base stat totals of all fully evolved Pokemon.
I cross-referenced with another page showing the length of evolution lines. For Pokemon with a recognized "Baby" stage on Bulbapedia I do not count the baby for determining the number of stages in their evolution. This means that for example, Blissey counts as a 2 stage evolution, not a 3 stage. I then calculate the average base stat total for Pokemon with the same number of stages.
I omit Mega and Primal Evolutions (which are not permanent/true evolutions), and only count the first entry for Pokemon with multiple forms (So Rotom, for example, will only be counted once, not 6 times)
As a side not, 1 stage Pokemon are the strongest, likely due to legendaries dominating this category.