Welcome to ribnet!
How much of a step change was it between OK & running like a bag of nails, and does it run like that both at idle and open throttle?
Thought process -
idle only, probbaly got crud in the idle jets.
wot (well, > about 2.5K), main jet probably blocked.
Assuming it's at all throttle positions, pull all 3 spark plugs - are any particularly wet & manky? if so you may have to adjust or worst case replace the float or the valve it closes. on the offending carb, however.....
Another thing to do (on grounds that it's an easy test and costs nothing) might be to swap your spark plugs & leads (or coils if they are the sealed units wuith short HT leads) around & see if the problem follows either the plug or the HT lead / coil. I've had 2 plugs die on me in almost as many years.