If all you have is a 3MM wick, you just need to adjust the hole as stated by Lucent Shadow.
I have made 3mm holes in Vivi heads and used 5mm cotton based wicks. If you have a 3mm wick, you need determine what the pinch point is for blocking good liquid flow through the wick. Obviously, your hole needs to be larger than the pinch point. You can approximate a pinch point by twisting your wick as tightly as possible. Measure the diameter of a tightly twisted wick, your hole need to be as large as the measurement. Different materials have different wicking capabilities so if may take a few try to optimize. So if your 3MM wick twists down to 2mm, start there and work your way up to 3MM. (I think the stock Vivi slot is only 1.5 MM.)
Take notes and let us know what worked for you.