Since the three countries that make up Allare were originally worlds themselves, they each had a king (or some such ruler). But when the three become one united world, they need a ruler to govern all three.

My question is, would that be wise? Or should they stick with each having a king, and cooperate. Or should they find someone to rule over them, so they answer to the head ruler?

And if they should have a 'high king', should it be one of them, or someone new, so as to prevent strife? What are your thoughts?

Well, I for my part would not advise my prince to surrender his sovereignty simply because his country now had neighbors. Maybe there's some special circumstance in your world that necessitates global government, but if not, my preference would be (if I lived in that world; I assume that's what you mean by "wise") for the countries to remain distinct and sovereign. Of course, from a plot perspective, it may be more useful to unite them, but that's a different issue.

I agree with Sam, and will embellish a bit. I think the nature of your government depends on the situation and the attitudes of your countries. I think, under normal circumstances, most countries would want to remain sovereign, although they might form a treaty. Especially if these countries were once separate worlds, they will have a very strong personal identity and probably won't want to sacrifice their government system, security, and way of life for a one-world government.

If, however, a war or outside force threatened them, they might want to band together more closely. You could also have some kind of radical political reform movement sweep through that would make the people want to unite. Maybe one country conquered the other two. If they did unite of their own free will, the nature of their collaboration would determine whether they set up a high-king or simply worked together. There are other possibilities, also - like sending ambassadors to a council, for example.

So, I would ask yourself - what kind of political arrangement do you need to make your story work? Figure out what you need, and then write backstory to make it happen. Alternately, if you don't need a particular system for a storyline, I would look at your current set-up and decide what kind of union (or lack thereof) was most reasonable.