This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / The Expendables
Billy's death, that is so excruciating and made quite one of the things that became personal for the team.
Tool's monologue in the original and Billy's in the second, when they both reveal what made them the shell shocked veterans they are now.
Tool: She was gone, and after takin' all them lives, here was one that I coulda saved, but I didn't. And what I realized later on was if I'da saved that woman, I mighta saved what was left of my soul.
There's a certain recognition of the passage of time at the very end of the sequel, when in an extremely meta moment, Barney comments on his new plane, "That belongs in a museum", to which Trench replies "We all do." A recognition that their age is coming towards an end.
In the third, Galgo's actual backstory is this. His former team was in a rescue mission similar to the one he and the Expendables are in, all went bad, and he was the only survivor.
Still in the third, Caesar being shot and the events that ensue, leading Barney to fire his team so that they will live. The latter part doubles as a moment of Heartwarming.
Doc being stunned that most of his comrades in the original team were long gone by the time Barney and the others saved him.
The short sequence in the third, where the older Expendables are shown listless and frustrated with the team disbanded and Barney working with the new blood, is surprisingly poignant. Without their shared purpose and camaraderie, Gunnar, Christmas, Toll Road and Doc just seem... lost.
Special mention to Doc, who's shown staring uncertainly at his motel room bed... before sleeping on a blanket on the floor next to it instead. Jars painfully with his earlier excitement at coming home.