For starters you shouldn't need any choke at all in this climate, it should only be needed in very cold weather. And i don't think i've ever actually ridden the bike with the choke still on, it must run like shite doing that alone?
How do you know it's misfiring on the 3rd cylinder?
First thing i would do would be to start with the basics.
New IRIDIUM plugs, they work much better than standard, they cost a few more quid but will last for many years.
If you want to maybe check the plugs first, swap 3 & 1 over and see if the issue moves with it.
Is the air filter good? Again i would personally change a standard one for a K&N, so a tad dearer but the last filter you'll ever buy for the bike as it's washable.