I don't own ARMA 3 but it says I does and I can't enter the giveaway because of that

Sync your account. here

Its because of the free weekend, steamgifts thinks that you actually own the full game, nothing that can be done about it if your account auto syncs while it lasts, also you might want to close this ones
http://www.steamgifts.com/forum/mV75O/i-dont-own-arma-3-but-it-says-i-does-and-i-cant-enter-the-giveaway-because-of-that
http://www.steamgifts.com/forum/BNABX/i-dont-own-arma-3-but-it-says-i-does-and-i-cant-enter-the-giveaway-because-of-that
http://www.steamgifts.com/forum/ILe2v/i-dont-own-arma-3-but-it-says-i-does-and-i-cant-enter-the-giveaway-because-of-that

4 topics not enough

You think someone won't see your topic? Create some more to be sure.

Sync your account. The game was in your library during the free weekend, that's why you are not able to enter.

Also, please don't hit the submit button multiple times when creating threads.