Trying to conceive for 12 months I had a three day period which usually last five day can I be pregnant.

It's possible. Some women will experience an unusual "period" in early pregnancy. Although not really an actual period, spotting or light bleeding around the time a period would be due is not uncommon. Of course, it could also have been just an off period.

Since you have been trying, take a pregnancy test.

Fingers crossed for you!