Translation:He has three older brothers.
It depends on who you ask, and what kind of robot, but I think the general consensus is います is for "animate" objects, meaning anything that can move through its own will/volition.
This means that you probably wouldn't use います for car manufacturing robots in an assembly line, but you might for more intelligent robots like Asimo. However, you could say that a Roomba "moves through its own volition", and thus you should use います for it, but that's why I said it depends on who you ask. Everyone draws the line in a different way.
It's a politeness thing, with おにいさん being more polite than あに. It (generally) depends on who you're talking TO and who you're talking ABOUT.
If you're talking TO your own older brother, you refer to him as おにいさん.
If you're talking TO someone ABOUT your own older brother, you refer to him as あに.
If you're talking TO someone ABOUT their older brother, you refer to him as おにいさん.
かれは - he/him (topic).
おにいさんが - older brother(s) (subject).
三人 - three people (人 here is the special suffix for counting people, pronounced にん when there are 3 or more). The count itself refers to the subject of the sentence (brothers in this case).
います - verb meaning "to exist" referring to animate objects, like people.
I'm mostly typing this out to break it down for myself but please correct me if wrong.
No, the plural form is かれら.
But keep in mind that 彼ら is more gender neutral than 彼. It can be used to refer to all male or mixed gender group.
And also be sure to remember that Japanese nouns, in general, can be either singular or plural. Plurality is not always specified. You just have to work it out from context.
So for example 犬 (いぬ) can mean dog or dogs.