Solution 1:

The word which means not complete is incomplete.

Although http://www.thefreedictionary.com/ defines uncomplete (adj) as not yet finished:
Ex: an uncompleted play .

the use of uncomplete as an adjective is very rare. I would suggest you to use incomplete or unfinished.

Solution 2:

The correct adjective is incomplete. The verb "uncomplete" does not exist. If you want to express that someone took something that had been complete and made it incomplete, you would have to say:

He removed the cog wheel from the engine, rendering it incomplete.

or

She erased her last answer (and now her application form is incomplete).

Solution 3:

Yes, it is a word, but it is uncommon. Less rare is the adjective "Uncompleted".