Is it possible to merge a mii created on a wii with one created on a 3DS?

It is possible to transfer from your ii directly to your 3ds by pressing a > b > 1 and then holding 2 until a secret menu pops up.

(It's like a cheat code. :P)

Reference Video Link

From the Wii to the Wii U it is simple enough to transfer the mii over via the Wii U's mii channel, that is if you've already transferred your data over from your Wii. It's a direct option in the mii transfer options.

Now since you already have a 3ds first you can synchronize your accounts on the Wii U with your 3ds making your profile mii the same on both systems.

Note that if you have network ID setup on the 3ds you'll can sync the profiles in order to have your eShop balance and wish list shared on both systems. It also will sync your Club Nintendo accounts as well.

Linking Club Nintendo account steps