If you are happy that it is not the Carbs and you say you have a weak spark in three then the most likely cause is corrosion within the plug cap and 3 is the most prone to it.
Remove the plug cap and test the resistance within it, It should be around 10Kohm but when corrosion gets in they can got to Megohms and the simplest solution to clean the internals.
The reason 3 is most prone becaue if on the side stand this hole fills with water and and does not always drain out the drain hole as the hole gets blocked
Looking in at the bottom of the plug cap you will see a slot for a screwdriver, unscrew this then a resistor, a tiny spacer (about 5mm diameter and verr thin) and a spring will fall out. If the spring does not come out it is stuck and will need a bit of twisting with a small screwdriver to dislodge it. Check the resistance of the resistor as this is where your 10kohm will be that can go faulty. On the screw in part, poslish off the corrosion off the flat end until you have shiny brass.. Then try to polish the spacer, if it breaks try and use something else as it saves the resistor from damage from the spring, but it will work without it for testing. Clean the spring and finally with a sharpened screwdrive or something like it clean/scratch the corrosion off the flat brass surface inside the plug cap.
Put it all back to gether and test it. It should now be around 10Kohm
Remove the plug cap and test the resistance within it, It should be around 10Kohm but when corrosion gets in they can got to Megohms and the simplest solution to clean the internals.
The reason 3 is most prone becaue if on the side stand this hole fills with water and and does not always drain out the drain hole as the hole gets blocked
Looking in at the bottom of the plug cap you will see a slot for a screwdriver, unscrew this then a resistor, a tiny spacer (about 5mm diameter and verr thin) and a spring will fall out. If the spring does not come out it is stuck and will need a bit of twisting with a small screwdriver to dislodge it. Check the resistance of the resistor as this is where your 10kohm will be that can go faulty. On the screw in part, poslish off the corrosion off the flat end until you have shiny brass.. Then try to polish the spacer, if it breaks try and use something else as it saves the resistor from damage from the spring, but it will work without it for testing. Clean the spring and finally with a sharpened screwdrive or something like it clean/scratch the corrosion off the flat brass surface inside the plug cap.
Put it all back to gether and test it. It should now be around 10Kohm