I can't decide on which dslr camera?

I've got about £300 to spent i have 3 cameras in mind but which is the best
1.sony alpha 300
2.canon 350d
3.nikon d40
i would prefer one with live view option.

Live view is a great idea in principle, but you'll find that you will rarely use it. The Canon 350d would be my choice.

I would say go for the Sony a300.It is a more advanced camera spec wise and offers a couple more features that the Canon and Nikon don't have.

Feature wise the a300 has:
- built in image stabilization (all lenses will be stabilized)
- quick AF live view
- tilting screen
- ability to use legacy lenses from Konica Minolta
- ability to attach a vertical grip

Based on specs the a300 has:
- higher sensitivity levels (3200 vs canon 1600)
- larger and higher resolution LCD screen
- more megapixels (may not be an advantage if megapixels don't matter to you)

There are more reasons but I can't think of any right now.

Seeing as you prefer one with live view, the Sony should be the stand out camera for you. Its quick and accurate and the camera as a whole offers a lot compared to the other two.