In simplest terms, you can't make Ubuntu just "take over" the Windows partition. Windows uses an NTFS filesystem, which Ubuntu does not support. (By default, Ubuntu uses ext4.) If you only want Mac OS X and Ubuntu, you'll have to first boot an OS live (your Ubuntu installation media is a good choice) and delete the Windows partition. You can then make your Mac OS X or Ubuntu partition (your choice) bigger to fill up the new free space you have. You can make Ubuntu bigger or Mac OS X bigger, just a preference. You will then have to see if you can boot Ubuntu still. (I'm assuming your dual-boot works by booting to GRUB and choosing what to boot from there.)
If you see your OS choices (it's okay if Windows is still there, we'll take care of that), choose "Advanced options" or something similar, and boot your latest Ubuntu kernel into recovery mode. You should see an option to Update GRUB. Choose that, and hopefully everything will work well with a reboot.
If the above worked for you, skip this. Now, let's suppose that booting your Mac didn't work. Maybe you got some "No bootable media" error or something, or GRUB brought you to some "GRUB recovery command-line", which, if you don't already know, is not the recovery mode I speak of above. We need to re-install GRUB (assuming your dual-boot works by booting into GRUB to choose your OS). Boot from your Ubuntu installation media, and open up GParted. Use this to figure out the path to your partition with Ubuntu. In my example, it will be /dev/sda2
. Once you figure that out, you can close GParted as we don't need that anymore, and open a terminal. Now, be careful when running this. I don't mean to alarm you, but if you don't install GRUB properly (i.e. you install it on top of Mac OS X), there could be disastrous effects! No pressure. Enter the command sudo grub-install /dev/sda2
, where /dev/sda2
is the path to your Ubuntu partition. Reboot and change your boot order to boot Ubuntu instead of any other OS. Hopefully, all should be well now. If it's not, look up your problem online. I'm not an expert on GRUB, and even if I was, making every possible error and problem a part of this answer would make an answer longer than it already is, so I am not going to help you with GRUB if you have problems after taking the advice I have already given you above.
ext4
.then runsudo update-grub
and boot again