[Eljaro]

that IS a big problem ! If you weld the holes shut, you will have to repair the throat bores and the may never be perfect. Drilling the holes on a flat surface is one thing , doing it on a welded uneven surface is another, your drill may wander inside and the hole may end up off its correct location.
You can try the epoxy way or plug all three holes with an aluminum shaft and then drill all three.
If all this does not help you will have to get a new carb.
Or return the bad one it to whomever you bought it from. If you can.