Do you think Apple should buy Microsoft,Sony and Motorola because:
1.They are all competitors
2.They can phoase out Windows
3.Moto makes cool phones
4.iPod lineup can grow by adding Sony mp3 players
I'll address your question in two ways: first addressing its possibilities, and then its merit.
Now, Apple is doing well, but it has nowhere near enough cash to buy Microsoft. We're talking about a company that is one of the largest non-oil companies around (the only two larger non-oil companies I can think of are GE and Citi). Simply put, Apple would have to somehow grow its own cash in order for this to happen, and that simply is not going to happen. Motorolla and Sony are still possible, but not likely because of the next question: the merits.
Now, your question asks if buying out the three mentioned companies would be a good idea because they are competitors. However, this isn't true in every aspect of their respective businesses. For example, while Microsoft and Apple compete on the OS front, they do not fight on the hardware front (with the minor exception of the Zune, which isn't really a viable threat to the iPod line right now).
Now, aside from this fact, buying out these companies would involve taking their weaknesses as well as their strengths. Motorolla, which was doing well when the RAZR craze hit, is now doing horribly. Its CEO recently announced his resignation, and the company is having trouble competing in the mobile phone industry because it was too aggressive in slashing its prices.
So, the short answer to your question is, NO, I don't think that buying out those companies would be a good idea.