It really depends on what you need to do. Travel a lot and need to read and write / edit long documents or manage large excel files? You will want a lightweight laptop.
Browse at home to read some headlines, emails or forums like this one? The tablet will be your companion. And you will use less the desktop or laptop.
Go out to see friends or for shopping and be able to access info stored on the Internet ? The tablet or a smartphone will be your choice.
You are stressed durig the week and wake up in the middle of the night? Instead of wandering to your pc you pick up your tablet (which is always on) and you will do the same stuff from your bed (dim down the light so you won't bother your significant other).
Need to do serious work like writing a lot, emailing a lot, video/photo editing, and all in a comfortable way with access to code for special characters? You will rather use a pc.
Of course it becomes problematic when you have use of all three types of devices, I mean moneywise. Every three years you will need to upgrade all you devices (maybe the desktop will survive a little longer as you can easily upgrade some components), but the whole thing can become expensive. In my case, for the work I do, tve frequent travel, etc. I am afraid I will have to keep all three devices...

Antonio