BF3 - When I fire a weapon my knife comes out and slashes can't play the game at all because sometimes it constantly does it and can't switch weapon.

On which platform are you playing BF3?

On PC.

Have you checked your key bindings under the options menu to make sure that LMB hasn't been re-bound to the "F" key? Are you using any mouse key binding software like Razer Synapse?

Yer key binding all good still using default and not using software because I'm using a sidewinder mouse that does have add on software but you don't have to use it because it's Microsoft it's pre built in drivers for there sidewinder gear.. Maybe the game is not properly recognising the sidewinder buttons because it has macro buttons and thumb buttons?

That is a possibility, Do you have a second mouse that you can test out to see if the problem replicates itself on another non-sidewinder piece of hardware?