In terms of premium currency and in-app purchases, the stores for the three games have already been disabled, but previously purchased premium currency can still be spent. Likewise, the games are no longer available to download on the App Store, so only current players can see the games through to their finales.
No specific reasoning was given for the closure of these three games, but we can only assume that it comes down to the almighty dollar (or in this case, the lack thereof). If more details are released about these three games in particular, we'll make sure to let you know.
Are you sad to see these three ngmoco games go, or had you already stopped playing them long ago? Sound off in the comments!