First off–if you want to use UE3 you would have to get special licensing from Epic, it uses several third-party technologies in it (which is why UE4 was created so they could remove those) and it has a much higher licensing cost due to things like that.
That being said, it’s probably not available for licensing anymore, I don’t know of any new licenses for UE3 for quite some time now, there’s some developers still using it but it’s a case where they were using it before UE4 was available and it would take too much time for them to switch, and usually they’ve implemented many of the improvements that UE4 has into their projects. (like Injustice 2 or Batman Arkham Knight)
In any case, most gaming hardware can run UE4, so you shouldn’t be concerned about people with low-end hardware not being able to play your game.