I'm taking allopurinol together with colchicine. If no more flare, can I drop colchicine but continously take allopurinol? Will gout attack reoccur?

Yes; possibly. You can stop Colchicine and use Allopurinol alone to prevent recurrent attacks. Keeping your Uric Acid level below 6 with Allopurinol and a low purine diet can reduce your likelihood of attacks in the future.
Yes you can! Allopurinol will keep your Uric acid level low and hence will prevent future gouty attacks. Keep Colchicine for treatment of the acute gouty attacks if they recur!
Possibly. Some people do well on just Allopurinol for suppressing gout attacks. Do not stop your medications without consulting your doctor first!