Hmm 3 pc's (oh wait do i see 4? ) - it's like having 4 engines in a car. One for each wheel..
...
Still i don't get it

Got some time today so hmm ok then I'll try to explain it to you.

1.
Would you, if you have only one car, use it for a track days and for rally racing and for shopping tours and for drag racing, etc.?
I'd say no as there is no such car the can be used (reasonable) for all these things, same with PCs.
There is no all in one wonder available for the needs I have and for the things I do/have to do with PCs.
2.
It's not that I bought 4 PCs at once to use them at the same time. I'm not that rich or nerdy.
It's more like I bought one of the best (at that time) 19" CRTs ten or eleven (I forgot) years ago for 500 Euros. But it is still in very good shape so what should I do? Sell it for 10 to 15 Euros on ebay (as you don't really get much for them today no matter if it is an Iiyama Vision Master that also has additional BNC connections (usefull if you need the right colours while doing gfx stuff that have to be printed later in a print company).
Same with the 17" TFT it was the worlds first 8ms TFT from BenQ and the price was ~550Euros.
But today I would hardly get 20 Euros for it if I sell it.
Same with the other stuff, ram,boards,cpu,gfx cards...
So before I throw it away for nothing (or almost nothing) I still use it where it makes sense.
A Backup system, a secured online banking PC etc.
If I can't really use it anymore then I give it away (often for free) to people who can use it or need it.
3.
You must see my complete desk more then a workbench then a usual room at home as I also work there on customers PCs or build them there.
I test their crashed HDDs or use my Monitor and Keyboard on their PCs, Save their data or check their stuff etc. So they usually only have to bring the PC nothing else (no keyb. no cable, no monitor). Very convenient for them.

Then on only one midi tower PC I couldn't record 3 different TV channels at the same time, download something with full speed and also play a new gfx intense game and also have enough space for backup Hdds and all this almost noiseless and without massive heat accumulation.
In the past 25 years I had 3 HDD crashes (physically dead not just a OS crash) and 2 times a bigger virus problem. So it's good (for me) to have some backup system available for the worst case.
As I also do webstuff, gfx stuff and adverts and lots of things like that on the PC so I always have to have at least one working PC around.
I couldn't tell my costumers that my work for them won't be ready because I have to install my PC again or change the HDD the next monday or when the new one will be delivered via mail.
Anyway I like to work on one PC and download something and follow the news on the 2nd screen while I record something in the living room on the 3rd. So for me it's a must to have more then one. It's my hobby and my job at the same time and maybe if I hadn't had my own Computer business for some years in another town then maybe I wouldn't had so much stuff here (as I only showed the 4 PCs

there are still more parts available here). But I was always a bit of a nerd I think since I first touched a computer a Zx81, then the Dragon64 the C64 then the Amiga 1000 and many others many many many years ago.

I wasn't unhappy when I only got one PC but during the years I wanted to do and I do so much different things with PCs that in the end the only sane solution for me is to have more then one.
Maybe now it's abit more understandable for you

If not np - to each his own
Last but not least I don't have a laptop
(I think because I hate laptops, netbooks, eeePCs etc. especially when you have to open or disassemble them)
When I travel I don't want to see computers around
but I got a windows OS mobile phone (but that doesn't count I think

)