Along a similar line as @kl78's answer, are they
Smartphone Operating Systems?
There's no one left but a group of 5
Windows, iOS, Android, Blackberry, and ?
Two of the group in an ongoing strife
Microsoft (Windows Phone) and Apple (iOS) have historically been "enemies" in so far as they were the major competitors in the battle for PC dominance.
The rejoicing third taking all the pride
Google (Android) was a newcomer to the OS scene, and yet Android has gained 80% market share. You could say they have the "lion's share" of the market, maybe where "pride" is pointing to.
Dear stranger, will you pick your side?
One who's fallen, no chance of return
All burnt out and down by the stern
Would probably refer to Blackberry, who has lost their once-ubiquitous place in the market. There is little chance for them to return to that status, and are shifting their strategy to more enterprise software.
Yet, another one hidden in the dark
Only waiting for this single spark
He's lying in wait in his covert hutch
This is where my answer is weakest (and thus likely wrong). This could refer to any of the tiny players in the mobile OS game. Sailfish is probably the newest and most probable, though historically this could point to Ubuntu or Firefox OS. Tizen is another possibility. "Spark" could refer to the need to "flash" the device's firmware to install these systems, as they can be put on consumer hardware but generally do not ship with it yet.
Merely longing for a finger's touch
"Finger's touch" would be literal here, because fingers are how you interact with your smartphone's touchscreen.