Hi Nikor,
Instead of waiting for the feature to be re-introduced or going back to the old version, why not try out an enhanced feature of Numbers 3 that is better than "auto-complete" in many situations where you frequently enter repeating values in a column and don't want to type the same text over and over again?
Try formatting the cells in the column with frequently repeating values as Pop-Up Menu.
In many situations, particularly if you want to avoid typos creeping in and messing up your data, Pop-Up Menu has clear advantages over "autocomplete." It is much enhanced over what it was in Numbers 2.3, and is easy to set up:

Select the whole column or range before formatting as Pop-Up Menu and your existing values in that column or range will pre-populate a menu automatically:

Then you can remove the values you don't want in the list, such as the column header (and possibly some previous spelling errors too!):

And after this easy one-time setup all you have to do thereafter is to choose from the pop-up list when you add rows:

See this post for more details on how this handy feature has been enhanced in Numbers 3.0.
In Numbers 3.0 there is no keyboard shortcut to activate the menu such as hitting the spacebar in the old Numbers (hope they'll add one in upcoming versions) but the Pop-Up Menu approach, unlike auto-complete, eliminates spelling inconsistencies or lack of capitalization when there should be capitalization, etc.
And should you ever want to take your Numbers with you and do data entry on an iPad or iPhone, in a touch interface pop-ups are much more efficient than typing the first few letters of items. You can scroll with ease through a list of dozens of items to make a pick.
See this thread for this and other workarounds for Numbers 3.0.
SG